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Phyllastraea tubitex; a, animal, partly expanded. 


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AND 


CORAL bhSiAN DS. 


BY 


JAMES D. DANA, aes 


PROFESSOR OF GEOLOGY AND MINERALOGY IN YALE COLLEGE; AUTHOR OF REPORTS IN 
CONNECTION WITH THE WILKES UNITED STATES EXPLORING EXPEDITION, 
ON GEOLOGY, ZOOPHYTES, AND CRUSTACEA; OF A SYSTEM OF 
MINERALOGY ; MANUAL OF GEOLOGY, ETC. 


Third Loition, 
WITH VARIOUS EMENDATIONS, LARGE ADDITIONS, THREE NEW 
MAPS, AND FOUR NEW COLORED PLATES. 


“ We wandered where the dreamy palm 
Murmured above the sleeping wave ; 
And, through the waters clear and calm, 
Looked down into the coral cave.” 


J. C. P., U.S. N. Expl. Expd. 


NEW. YORK: 
DODD, MEAD, AND COMPANY, 


753 AND 755 BROADWAY. 


Entered according to Act of Congress, in the year 1872, by 
JAMES D. DANA, 
in the Office of the Librarian of Congress, at Washington. 


Copyright, 1890, 
By Dopp, MEAD, AND COMPANY. 


Gniversity Yress : 
JouNn WILSON AND SON, CAMBRIDGE. 


PREFACE TO THE THIRD EDITION. 





HIS third edition of the “Corals and Coral Islands” 
contains a full discussion of the views and arguments 
which have recently been brought forward in opposition to 
the theory of coral reefs proposed by Darwin and explained 
in the following pages. Besides this, changes and additions 
have been made in all parts of the work in order to bring 
it up to date of publication. Moreover, four new maps have 
been introduced: one, of the Central Pacific ; the second, of 
the large coral-reef region of the Louisiade Archipelago, in 
the southwest Pacific; the third, a new map of the Florida 
and Bahama coral-reef banks, from the charts of the United 
States Hydrographic Department ; and the fourth, a copy of 
part of the Hawaiian Government map of the vicinity of 
Honolulu, showing the coral reefs off the shores, and the 
positions of the many artesian borings on this border of Oahu 
that are throwing light on the thickness of the shore reef. 
The work has its value enhanced also by four new colored 
plates, one representing Actinias of the Pacific, and the others 
living corals, selected from the author’s Atlas of his Explor- 


ing Expedition Report on Zodphytes. 


JAMES D. DANA. 


New Haven, Cr., 
February 12, 1890. 


eee ivak 
Be 





PREFACE TO THE SECOND EDITION. 


In the preparation of this work fora second edition, a few 
emendations have been made, and new facts introduced from 
recent publications on the subject. Among the latter, an 
account is given of the arrangements by MM. Le Clere and De 
Benazé, on Tahiti, for marking, in the future, the rate of 
growth of the Dolphin Shoal or reef. 


The Preface to the first edition alludes to some points 
in which the author differs from Mr. Darwin. The reader 
will find some additional remarks on these differences in the 
American Journal of Science for October, 1874, called out 
by the discussion of the subject in the new edition of Mr, 


Darwin’s “ Coral Reefs.” 


NEw HAVEN, Conn., October 1, 1874. 








PRHEFACH. 


HE object in view in the preparation of this work has been 
to present a popular account of “Corals and Coral 
Islands,” without a sacrifice of scientific precision, or, on the 
main topic, of fulness. Dry details and technicalities have 
been avoided so far as was compatible with this restriction, 
explanations in simple form have been freely added, and 
numerous illustrations introduced, in order that the subject 
may have its natural attractiveness to both classes of readers. 
I have opened the volume with a chapter on “ Corals and 
Coral makers,” describing, under it, the forms and structure 
of polyps; how they live and grow and hold their own in a 
world of enemies; how coral-making species secrete their 
coral; how they multiply, and develop their large clusters, 
spreading leaves and branching forms, so much like those 
among plants; and in what seas they thrive, and under what 
conditions produce the coral plantations, 
All this is prefatory to the following part of the volume on 
Coral Reefs and Islands, which comprises a description of the 
features and structure of these reef-formations, an account of 
~ their mode of accumulation and increase, and a discussion of 


4 PREFACE. 


the origin of the included channels and lagoons, and of the 
distribution of reefs, together with a review of the facts with 
reference to their geological bearing. 

The observations forming the basis of the work were made 
in the course of the cruise of the Wilkes Exploring Expedi- 
tion, around the world, during the four years from 1838 to 
1842. The results then obtained are published in my Report 
on Zodphytes, which treats at length of Corals and Coral 
Animals, and in a chapter on Coral Reefs and Islands form- 
ing part of my Geological Report. 

The opportunities for investigations in this department. 
afforded by the Expedition, were large. We visited a number 
of the coral islands of the Paumotu Archipelago, to the north 
of east from Tahiti; also, some of the Society, Navigator, and 
Friendly Islands, all remarkable for their coral reefs; the 
Feejee Group, one of the grandest regions of growing corals 
in the world, where we spent three months; several islands 
north of the Navigator and Feejee Groups, including the Gil- 
bert or Kingsmill Group; the Sooloo sea, between Borneo 
and Mindanao, abounding in reefs; and, finally, Singapore, 
another East India reef-region. 

Most agreeable are the memories of events, scenes and 
labors, connected with the cruise:—of companions in travel, 
both naval and scientific; of the living things of the sea, 
gathered each morning by the ship’s side, and made the study 
of the day, foul weather or fair; of coral islands with their 
groves, and beautiful life, above and within the waters; of 
exuberant forests, on the mountain islands of the Pacific, 
where the tree fern expands its cluster of large and graceful 
fronds in rivalry with the palm, and eager vines or creepers 
intertwine and festoon the trees, and weave for them hangings 
of new foliage and flowers ; of lofty precipices, richly draped, 


PREFACE. ‘ 5 


even the sternest fronts made to smile and be glad as delights 
the gay tropics, and alive with waterfalls, gliding, leaping, or 
plunging, on their way down from the giddy heights, and, 
as they go, playing out and in amid the foliage; of gorges ex- 
plored, mountains and volcanic cones climbed, and a burning 
crater penetrated a thousand feet down to its boiling depths; 
and, finally,—beyond all these.—of man emerging from the 
depths of barbarism through christian self-denying, divinely- 
aided, effort, and churches and school-houses standing as cen- 
tral objects of interest and influence in a native village. 

On the other hand, there were occasional events not so 
agreeable. 

Even the beauty of natural objects had, at times, a dark 
back-ground. When, for example, after a day among the 
corals, we came, the next morning, upon a group of Fee 
jee savages with human bones to their mouths, finishing off 
the cannibal feast of the night; and as thoughtless of any im- 
propriety as if the roast were of wild game taken the day be 
fore. In fact, so it was. 

Other regions gave us some harsh scenes. One—that of 
our vessel, in a tempest, fast drifting toward the rocks of 
Southern Fuegia, and finding anchorage under Noir Island, 
but not the hoped-for shelter from either winds or waves; the 
sea at the time dashing up the black cliffs two and three hun- 
dred feet, and shrouding in foam the high rocky islets, half- 
obscured, that stood about us; the cables dragging and clank- 
ing over the bottom; one breaking; then another, the storm 
still raging; finally, after the third day, near midnight, the 
last of the four cables giving way, amid a deluge of waters 
over the careering vessel from the breakers astern, and an in- 
stant of waiting among all on board for the final crash; then, 
that instant hardly passed, the loud calm command of the 


6 PREFACE. 

Captain, the spring of the men to the yard-arms, and soon the 
ship again on the dark, stormy sea, with labyrinths of islands, 
and the Fuegian cliffs to leeward; but, the wind losing some- 
what of its violence and slightly veering, the ship making a 
bare escape as the morning dawned with brighter skies. 

And still another scene, more than two. vears later, on a 
beautiful Sunday, in the summer of 1841, when, after a cruise 
of some months through the tropics, we were expecting soon 
to land on the shores of the Columbia; of the vessel sud- 
denly stopped on the grinding sands; there, as the waves 
passed, rising and falling with heavy blows on the fatal 
bar that made the timbers to quiver and creak; and thus 
helpless through a long night, the waters gaining in spite 
of the pumps;— morning come, the old craft, that had 
been a home for three eventful years, deserted, the boats 
carrying us, empty handed, to ‘Cape Disappoimtment ’ — 
a name that tells of other vessels here deceived and wrecked ; 
and, twenty hours later, the old “ Peacock” gone, her upper 
decks swept off by the waves, the hulk buried in the sands. 

But these were only incidents of a few hours in a long and 
always delightful cruise. If this work gives pleasure to any, 
it will but prolong in the world the enjoyments of the “ Ex- 
ploring Expedition.” 

In explanation of some allusions in the following pages, 
I may here state with regard to the Exploring Expedi- 
tion, that Captain (now Admiral) Cuartes Wixkes, U.S. N., 
the Commander of the Expedition, was in charge of the Sloop- 
of-war Vincennes; Capt. Wm. L. Hupson, U. 8. N., of the 
Sloop-of-war Peacock; Capt. A. K. Lone, U. 5S. N., of the 
Storeship Relief (the vessel which encountered the dangers 
in the Cape Horn sea, above related) ; and Lieut. Command- 
ant C. Rineeorp, of the Brig Porpoise; and that my associates 


PREFACE. 7 


in the ‘Scientific Corps” were Dr. Cuaries Pickertna, J. P. 
Cournouy, and Trrtan R. Prax, Zodlogists; Wu. Ricu and 
J. D. Brecxenripe®, Botanists; Horatio Hare, Philologist; 
JosEpH Drayton and A. T. Acars, Artists. 

Our cruise led us partly along the course followed by Mr. 
Cuarurs Darwin during the years 1831 to 1836, in the Voyage 
of the Beagle, under Captain Firzroy ; and, where it diverged 
from his route, it took us over scenes, similar to his, of coral and 
voleanic islands. Soon after reaching Sydney, Australia, in 
1839, a brief statement was found in the papers of Mr. Dur- 
win’s theory with respect to the origin of the atoll and barrier 
forms of reefs. The paragraph threw a flood of light over the 
subject, and called forth feelings of peculiar satisfaction, and of 
gratefulness to Mr. Darwin, which still come up afresh when- 
ever the subject of coral islands is mentioned. The Gambier 
Islands, in the Paumotus, which gave him the key to the 
theory, I had not seen; but on reaching the Feejces, six’ 
months later, in 1840, I found there similar facts on a still 
grander scale and of more diversified character, so that I was 
afterward enabled to speak of his theory as established with 
more positiveness than he himself, in his philosophic caution, 
had been ready to adopt. His work on Coral Reefs appeared 
in 1842, when my report on the subject was already in man- 
uscript. It showed that the conclusions on other points, which 
we had independently reached, were for the most part the 
same. The principal points of difference relate to the reason for 
the absence of corals from some coasts, and the evidence there- 
from as to changes of level, and the distribution of the oceanic 
regions of elevation and subsidence—topics which a wide 
range of travel over the Pacific brought directly and constantly 
to my attention. 

In the preparation of the present work my former chapter 


8 PREFACE. 


on Coral Reefs and Islands has been greatly extended by the 
addition of facts from numerous sources. The authorities 
cited from are stated in the course of the volume, and need 
not here be re-mentioned. I have occasion, however, for special 
acknowledgments to our excellent Yale Zodlogist, Professor 
A. E. Verrini, who now stands first in the country in the de- 
partment of Zoéphytes. Through his recent memoirs on the 
subject, and also by his personal advice, I have been greatly 
aided in acquainting myself with the present state of the sci- 
ence :—my own special labors in this branch of zoélogy having 
ended in 1850, when both the Reports, referred to above, had 
been published, and the last of my Expedition departments— 
that of the Crustacea—forced my studies in another direction. 

The illustrations of the following pages have been drawn 
mainly from my Expedition Reports. Those not my own are 
from the works or memoirs of Gossr, Ménrus, VerriLL, Pour- 
TALES, L. Acassiz, A. Acassiz, Swirr, Enwarps and Hare, 
Wixes, and Harrr. In addition, the volume is indebted for 
a few cuts to the beautifully illustrated popular works, ‘ Le 
Monde du Mer” and “ La Vie et les Mceurs des Animaux;” 
but nearly half of these were engraved from my plates. The 
sources of all the figures are given in the List of Illustrations. 


James D, Dana. 


New Haven, Conn., February 12, 1872. 


CON DHOIN TS. 


CHAPTER I. 


CORALS AND CORAL MAKERS. 


Page 

PNM ALORA EVA TIONS (4. 0* Rk ae sek ui oes te. Wt eee) ecltes ee Gl 

PPA Ok ee eet ey es Wa Laine kee ay allele. ta ey QO 

I. Actinoid Polyps . “ : P| 

I. Non-Coral-Making hee : 21 

II. Coral-Making Actinoid Polyps 49 

III. Classification : Bg EEG LH Peis ei) om pea ed 

II. Cyathophylloids, or Rugosa Tetr Pana SM Ed oe on Wher Lio i's riers 78 

III. Alcyonoid Polyps .. . SU ACA komt ORES BOC. Oe 

IV. Life and Death in Gace ee SBN Matic a yioN Koo eel atte bet ioe oe 

We omposition oi Coralsi eyes ce) ers Pease he! fs) So velit ey oe 298 

ieLeyD ROU Si eerat sakes }- a TNL Pee nem Ls Bie ah lat wo le) toe Wl Oil 

Ra COZOANE Ci, ie ee Se ee ee ee et eb he em ae LOD 

IV. NuLirporrs. . ree: ie ee seated ss) et, at a es LOE 
V. Tue Reer-FormMING Caacs, AND THE CAUSES INFLUENCING THEIR 

Crowley AND DISTRIBUTION: ia cs le cals) 2 es ate ws LOS 

ia Distnibusoncin Watitider meer ls Mares ee el e's Ye LOS 

II. Distribution in Depth . . . . RSS SS ae were cs 2) 2 C 


III. Local Causes influencing Distribution SURE egc cng stank = oe, va)’ Vee LO 
iver hats. oimGrowinOr WOralsecl oie We ck te este ge seeds 


CHAPTER II. 


STRUCTURE OF CORAL REEFS AND ISLANDS. 


IL (CRS IMSISED Gob Ge Ses a Es Se a a em ccs bf) 


GSnera lehleauurese meet a ae vont aa wee raat eet yikes eee 198 
MORON CTM CDi ae eer etl mE ra kd) Cea We Po S186 


10 CONTENTS. 


Ill. Formations in the Sea outside of Barrier Reefs . 
IV. Inner Reefs. 
V. Channels among Reefs . 
VI. Beach Sand-Rock 
VIL. Drift Sand-Rock . 
VIII. Thickness of Reefs ; 
IX. A Good Word for Coral Reefs 


II. Cora IsLaAnps 


I. Forms and General Features . 

II. Soundings about Coral Islands 

III. Structure of Coral Islands . 

IV. Notices of some Coral Islands 
Maldive Archipelago 
Great Chagos Bank . j 
Metia, and other elevated stands 
Birnie’s, Enderbury’s 
Hall’s, Swain’s 
Oatafu, Fakaafo . : 
Washington, Otuhu, Mocseee Teku, aes 
Taiara, Ahii 
Raraka . 
Kawehe . Pea Se, we 
Manhii, Aratica, Nairsa or Dean’s . 
Florida Reefs and Keys 


Between the Florida Reefs and Gules Salt ae Hane 


Bahama Islands 
Bermuda or Somers’ Islands . 


CHAPTER III. 


PAGE 
139 
144 
148 
152 
154 
156 
159 


161 


161 
Wel 
174 
185 
186 
192 
193 
196 
197 
198 
199 
200 
201 
202 
203 
204 
210 
214 
218 


FORMATION OF CORAL REEFS AND ISLANDS, AND CAUSES OF THEIR 


FEATURES. 


I. ForMATION OF REEFs . 


I. Origin of Coral Sands and the Reef-Rock 
II. Origin of the Shore Platform . 
Ill. Effects of Winds and Gales 
Il. Causes Mopiry1nc THE Forms AND GROWTH OF REEFS 
I. Barrier and Fringing Reefs 
II. Atoll Reefs . : ; 
Ill. Rate or GrowtTH oF Ree: 


227 
224 


CONTENTS. 11 


Pace 

IV. ORIGIN OF THE BARRIER CONDITION OF REEFS, AND OF THE ATOLL 
HGEMIORICORALCISLANDS): (ia fii. Bowe) es Bo. Aloe |. 258 
be Old. Vidws: i « mele 258 
II. Darwin’s Theory of the ovina of Baitions ana ‘Aechii diel fet AOR 
III. Objections to the Subsidence Theory . . . . . . . . . 277 
MebnEeOComprernD  ATOLE «<<... 0-2 Se 6. 8 a ew ee s. BOD 


CHAPTER IV. 


GEOGRAPHICAL DISTRIBUTION OF CORAL REEFS AND ISLANDS. 335 


CHAPTER V. 


CHANGES OF LEVEL IN THE PACIFIC OCEAN, 


I. Evidences of Change of Level. . . . Eat ta 2st Prat Nal) oe oO: 
II. Subsidence indicated by Atolls and Baeeiee Reefs By te Bn mh A re eens OT 
Bbeerbitect.of the Subsidence =. . . 2 (fies. 6 + ties ws «O06 
IV. Period of the Subsidence . . . SORA, Picea, vot, en ee eerO 
VY. Elevations of Modern Eras in the Pacific Sra batetss ct cart ee Vawe eat er OOS 


CHAPTER VI. 


GEOLOGICAL CONCLUSIONS. 


SCHORMATION ORULIMESTONES Ys oe.) a ss se 8 lw Ud te BBB 
II. Beps or LimMEsTONE witH Livinc MarGins ..... . . 9887 
TI. Maxine or Tuick STRATA OF LIMESTONE. ...... =. 938 
IV. SuBSIDENCE ESSENTIAL YO THE MAKING OF THICK STRATA. . . 387 
V. Deep-Sea LIMESTONES SELDOM MADE FROM CoRAL ISLAND OR 
levaioims IDR aah Sn RBU Se Vy lee SO Oo ane ee Retr foo 
VI. ABSENCE oF FossILs FROM LIMESTONE STRATA ..... . 9899 
VII. THe wipE RANGE OF THE OLDER LIMESTONES NOT EXEMPLIFIED 
IN MopERN CORAL-REEF FORMATIONS ... -..- - 999 
WITI. CoNSOLIDATION OF CoRAL Rocks . .... .; BESS aes sen 0) | 
IX. Formation oF DOLOMITE OR MAGNESIAN Cee oF LIME . 393 
Moe NORMATIONGONS OHIATIKCn er yes Ny ee) fae ee ge BOA 
XI. Rate or IncREASE OF LIMESTONE FORMATIONS . .. .. -. .9896 
PU IMUSTONM CA VINE NS ONG cys os) cy we Nelle) ey ese! we ote OOF 
Mele tOCKANIC LHMPERATURE |  . «= ¢4.. Bre cs Cera aati euimermane 1210) 


Miva Lun OCHANIC) CORAL-ISLAND SUBSIDENCE. . «= «|. . . «+ 401 


12 CONTENTS. 


APPENDIX. 


i. ARTESIAN WELLS ON SOUTHERN OAHU . : 
Il. Rare or GrRowTH OF CORALS AND CORAL REEFs . 


lll. NAMEs oF SPECIES IN THE AUTHOR’S REPORT ON ZOOPHYTES . 


INDEX . 


PaGe 


411 
417 
420 


431 


LIST OF ILLUSTRATIONS. 


Tue following list contains a statement of the original sources of the illustra- 
tions through the volume. By ‘‘ Author’s Atlas’’ is to be understood the Atlas 
of his Report on Zoophytes. The figures are of natural size, except when other- 
wise stated. The new figures included have been made by Mr. Lockwood San- 
ford, a New Haven wood-engraver of most of the wood-cuts in this volume. 


I. PLATES. 


Plate I., frontispiece. Fig. 1, la. Phyllastreea tubifex. Author’s Zoodphyte 
Atlas, Plate 16. 

II., facing page 20. Fig. 1, Phymactis veratra: Actinia veratra, Author’s 
Zoophyte Atlas, Plate 1. Fig. 2, Phymactis clematis: Actinia clem- 
atis, Ibid., Plate 1. 

VIII, page 31. Lasso-cells, K. Mobius, Abh. Nat. Ver. Hamburg, vol. v., 
1866. 

IV., facing page 54. Fig. 1, Caulastraea furcata. Zoophyte Atlas, Plate 9. 
Fig. 2, Tridacophyllia peonia, Ibid. ‘Fig. 3, Leptoria tenuis: Me- 
andrina tenuis, Ibid., Plate 14 ; the tentacles are arranged along the 
sides of the trench, with the mouths of the polyps between them. 

V., page 73. Madrepora formosa. Zoophyte Atlas, Plate 58. 

‘VL, facing page 82. Fig. 1,15. Merulina regalis. Zoophyte Atlas, Plate 15. 
Fig. 2, Telesto trichostemma: Gorgonia trichostemma of Zoophyte 
Atlas, Plate 59; referred to Telesto by Verrill. 

VII., page 133. Louisiade Group. Proceedings of the R. Geograph. Soc., 
Sept., 1889. 

VIII, page 165. Gilbert or Kingsmills Group. Author’s Expl. Geol. Rep. 
from Expl. Exped. Maps. 

IX., facing page 172. Phoenix Group. U. S. Hydrographic Maps of the 
Pacific. 

X., page 187. Maldive Archipelago. Darwin on Coral Reefs. 

XI., facing page 204. Map of Florida, Bahamas, and Bermudas; U. S. 
Hydrographic Maps of the Atlantic. 

XIL.,. facing page 262. Map of the Feejee Islands. Wilkes’s Narrative of 
the Expl. Expedition. 

XIII., facing page 310. Cocoanut Grove, on Bowditch Island. Wilkes’s Nar- 
rative, vol. v. 


14 


LIST OF ILLUSTRATIONS. 


Plate XIV., facing page 314. Village of Utiroa. Wilkes’s Narrative, vol. v. 
XV., facing page 312. Scene on the Lagoon side of Oatafu or Duke of 


York’s Island. Ibid. 


XVI, atend. Isocrymal Chart of the Oceans. Author’s Expedition Report 


Page 23, 


24, 


26, 


27, 


42, 


43, 


45, 


~~ 


46, 


AT, 


69 


~ 


on Crustacea. 


II. FIGURES IN THE TEXT. 


Paractis rapiformis. From a drawing by the Author, made in 1852. 
Cancrisocia expansa. Verrill, Amer. Naturalist, from Proc. Essex In- 


stitute, vol. vi. 
fig. 1. Peachia hastata. Gosse’s Actin. Brit., Plate viii. 


fig. 2. Edwardsia callimorpha; and 3. Halocampa chrysanthellum. Ibid., 
Plate 7. 

Section of Actinia. From a drawing by the Author, made in 1856, on 
the basis of a study (1852) of the Actinia figured on p. 23. 

Caryophyllia cyathus. Le Monde du Mer. 

Thecocyathus cylindraceus. Pourtales on Deep-Sea Corals, Plate 2. 

Flabellum spheniscus. Euphyllia spheniscus of Author’s Atlas, Plate 6. 

Ctenactis echinata, one-third natural size. La Vie et les Mceurs des 
Animaux. 

Fungia lacera, living and expanded. F. echinata of Author’s Atlas, 
Plate 18. 

Enlarged view of tentacle of F. lacera, and profile, natural size, of one 
of the calcareous septa. Ibid., Plate 18. 

Madrepora aspera, living and expanded. Author’s Atlas, Plate 38. 

Dendrophyllia nigrescens, living and expanded. Author’s Atlas, 
Plate 30. 

Goniopora columna. Ibid., Plate 56. 

Porites mordax. Ibid., Plate 53. 

Cladocora arbuscula. Caryophyllia arbuscula of Ibid., Plate 27. 

Orbicella cavernosa. L. Sanford, from specimen. 

Spontaneous fission. Author’s Report on Zoophytes. 

Astrea pallida. Author’s Atlas, Plate 10. 

Epizoanthus Americanus. Verrill, Amer. Naturalist, vol. iii., p. 248. 
View of single polyp. From a drawing by Prof. Verrill. 

Antipathes arborea, with enlarged view of polyp. Author’s Atlas, 
Plate 56. 

Astrea pallida. Author’s Atlas, Plate 10. 

Diploria cerebriformis. Le Monde du Mer. 

Fungia Dane. L. Sanford, one-sixth the natural size. From a photo- 
graph by Prof. A. E. Verrill. 

Caryophyllia Smithii, one of the figures with the animal expanded; the 
other with it contracted. Gosse’s Actinologia Britannica, Plate 10. 
Astrangia Dane ; fig. a. one of the polyps enlarged; c. coral with the 

polyps expanded, natural size. Agassiz, Seaside Studies. 
fig. b. surface of corallum, natural size. L. Sanford, from specimen. 
Phyllangia Americana, Florida. Edwards & Haime, Corallieres. 


LIST OF ILLUSTRATIONS. 15 


Page 69, fig- 1. Oculina varicosa, extremity of a branch. Author’s Report on 
Zoophytes, page 67, corrected from specimen. 

fig. 2,3. Stylaster erubescens; 2. corallum, natural size; 3. extremity of 
a branch enlarged. Pourtales, Deep-Sea Corals. 

fig. 4, 5. Stylophora Danex; 4. extremity of a branch ; 5. one of the 
calicles enlarged. Sideropora palmata of Author’s Atlas, Plate 49. 

fig. 6. Polyp, enlarged, of St. mordax. Author’s Atlas, Plate 49. 

fig. 7. Pocillipora grandis. L. Sanford; from an Exploring Expedition 
specimen; portion of one of the large, flattened branches of the coral- 
lum. An entire clump is figured in the Author’s Atlas, Plate 51. 

fig. 8. Cell, enlarged, of Pocillipora elongata. Author’s Atlas, Plate 50. 

fig. 9. Cell, enlarged, of Pocillipora plicata. Ibid., Plate 50. 

fig. 10. Vertical section of corallum of P. plicata, showing the tabular 
structure. Ibid., Plate 59. 

72, Polyp of Madrepora cribripora, enlarged. Author’s Atlas, Plate 31. 

75, Polyp of Dendrophyllia nigrescens, enlarged. Ibid., Plate 30. 

76, Dendrophyllia nigrescens, natural size. Ibid., Plate 30. 

77, Alveopora Verrilliana, natural size; the corallum covered below with a 
peritheca. Alveopora dedalea in part of Author’s Atlas, Plate 48. 
The species is here named after Prof. A. E. Verrill, as it is not the 
true A. dedalea. 

Alveopora spongiosa, vertical section of corallum, and upper view of 
calicle, much enlarged; the diameter of the cell being about a 
fifteenth of an inch. Author’s Atlas, Plate 48. 

78, Polyp of Porites levis, enlarged. Author’s Atlas, Plate 54. 

79, Porites levis, with the polyps of one of the branches expanded, natural 
size. Author’s Atlas, Plate 54. 

82, Xenia elongata. Author’s Atlas, Plate 57. 

83, Anthelia lineata. Verrill, Proceedings of the Essex Institute, vol. iv., 
Plate 5. From a drawing by Dr. Stimpson. 

84, Telesto ramiculosa. Verrill, Proc. Essex Inst., vol. iv., Plate 6; the 
second figure, an enlarged view of expanded polyp. From drawings 
by Dr. Stimpson. 

Tubipora syringa; fig. 1. part of a clump, natural size; 2. one of the 
polyps expanded. Author’s Atlas, Plate 59. 

Tubipora fimbriata (3d figure), polyp, expanded. Author’s Atlas, 
Plate 59. 

85, Gorgonia flexuosa, part of zoophyte, natural size. Author’s Atlas, 
Plate 60. 

86, Spicules of Gorgoniw, much enlarged. Verrill, Transactions of the 
Connecticut Academy of Sciences, vol. i., Plates 4 and 5. 

88, Isis Hippuris. La Vie et les Moeurs des Animaux. 

89, Corallium rubrum, the coral, natural size. L. Sanford, from specimen. 

Extremity of branch of C. rubrum, enlarged, with some of the ani- 
mals expanded. Lacaze-Duthiers, from La Vie et les Mceurs des 
Animaux. 

91, Cophobelemnon clavatum: the small figure, enlarged view of one of the 
polyps. Verrill, Proc. Essex Institute, vol. iv., Plate 5. From a 
drawing by Dr. Stimpson. 


16 
Page 91, 


95, 
101, 
103, 
104, 


105, 
106, 


130, 


140, 
149, 


162, 
168, 


170, 


176, 
179, 


189, 


191, 
192, 
193, 


219, 
235, 
943, 


247, 
248, 
250, 
263, 
264, 
266, 
267, 
268, 
311, 
413, 


LIST OF ILLUSTRATIONS. 


Veretillum Stimpsoni, enlarged three diameters. Verrill, Proc. Essex 
Institute, vol. iv.. Plate 5. From a drawing by Dr. Stimpson. 

Caulastrea furcata. Author’s Atlas, Plate 9. 

Hydra. Le Monde du Mer. 

Hydrallmania faleata. Le Monde du Mer. 

Animals of M. alcicornis, enlarged. L. Agassiz, Contributions to the 
Natural History of the United States, vol. iii. Plate 15. 

Millepora alcicornis. La Vie et les Mceurs des Animaux. 

Hornera lichenoides: 1. natural size; 2. part of branch enlarged. 
Smitt’s Mém. des Bryozoaires. 

Discosoma Skenei, part of a group much enlarged. Ibid. 

High Island, with Barrier and Fringing Reefs. Author’s Exp. Geol. 
Report. 

The Lixo Coral Reef, Abrolhos. Hartt’s Brazil, p. 202. 

Coral Reefs off the North Shore of Tahiti. Author’s Exp. Geological 
Report, from the Wilkes Expl. Exp. Maps. 

Coral Island or Atoll. Wilkes’s Narr. Expl. Exped. 

Maps of Taiara, Henuake, Swain’s Island, Jarvis Island, and Fakaafo. 
Author’s Geol. Rep.; from Exp]. Exp. Maps. 

Map of Menchicoff Atoll. Darwin on Coral Reefs; from Kotzebue’s 
Atlas. 

Section of the rim of an Atoll. Author’s Exp. Geol. Report. 

Blocks of Coral on the shore platform of Atolls, Author’s Exp. Geol. 
Report. 

Map of Mahlos Mahdoo Atoll, one of the Maldives. Darwin on Coral 
Reefs. 

Map of Great Chagos Bank, Darwin on Coral Reefs. 

East and West Section across the Great Chagos Bank. Ibid. 

Metia, an elevated Coral Island. Wilkes’s Narrative of Expl. Exp., 
vol. i. 

Map of the Bermuda Islands; reduced from an English Chart. 

The ‘‘Old Hat.’’ Author’s Exp. Geol. Report. 

Harbor of Apia. Author’s Exp. Geol. Rep.; from Charts of the Wilkes 
Expl. Exped. 

Part of North Shore of Tahiti. Ibid. 

Harbor of Falifa. Ibid. 

Whippey Harbor. Ibid. 

Section illustrating the Origin of Barrier Reefs. Ibid. 

Map and Ideal Section of Aiva Island. Ibid. 

Map of Gambier Islands. Darwin on Coral Reefs. 

Section illustrating the Origin of Atolls. Author’s Exp. Geol. Rep. 

Menchicoff Atoll. Darwin on Coral Reefs. 

Fakaafo. Author’s Exp. Geol. Rep.; from Charts of Expl. Exped. 

Map of part of Oahu; reduced from chart of Hawaiian Government 
Survey. 


CORALS AND CORAL ISLANDS. 





CHAPTER I. 
CORALS AND CORAL MAKERS. 


SINGULAR degree of obscurity has possessed the popu- 
lar mind with regard to the growth of corals and coral 
reets, in consequence of the readiness with which speculations 
have been supplied and accepted in place of facts; and to the 
present day the subject is seldom mentioned without the qual- 
ifying adjective mysterious expressed or understood. Some 
writers, rejecting the idea which science had reached, that 
reefs of rocks could be due in any way to ‘ animalcules,” 
have talked of electrical forces, the first and last appeal of ig- 
norance. One author, not many years since, made the fishes 
of the sea the masons, and in his natural wisdom supposed 
that they worked with their teeth in building up the great 
reef. Many of those who have discoursed most poetically on 
zodphytes have inagined that the polyps were mechanical 
workers, heaping up the piles of coral rock by their united la- 
bors; and science is hardly yet rid of such terms as polypary, 
polypidom, which imply that each coral is the constructed 
hive or house of a swarm of polyps, like the honey-comb of 
the bee, or the hillock of a colony of ants. 


Science, while it penetrates deeply the system of things 
2 


18 CORALS AND CORAL ISLANDS. 


about us, sees everywhere, in the.dim limits of vision, the word 
mystery. Surely there is no reason why the simplest of organ- 
isms should bear the impress most strongly. If we are aston- 
ished that so great deeds should proceed from the little and 
low, it is because we fail to appreciate that little things, even 
the least of living or physical existences in nature, are, under 
God, expressions throughout of comprehensive laws, laws that 
govern alike the small and the great. 

It is not more surprising, nor a matter of more difficult 
comprehension, that a polyp should form structures of stone 
(carbonate of lime) called coral, than that the quadruped 
should form its bones, or the mollusk its shell. The pro- 
cesses are similar, and so the result. In each case it is a sim- 
ple animal secretion ; a secretion of stony matter from the 
aliment which the animal receives, produced by the parts of 
the animal fitted for this secreting process; and in each, car- 
bonate of lime is a constituent, or one of the constituents, of 
the secretion. 

This power of secretion is then one of the jist and most 
common of those that belong to living tissues ; and though dif- 
fering in different organs according to their end or function, it 
is all one process, both in its nature and cause, whether in the 
Animalcule or Man. It belongs eminently to the lowest kinds 
of life. These are the best stone-makers ; for in their simplici- 
ty of structure they may be almost all stone and still carry on 
the processes of nutrition and growth. Throughout geological 
time they were the agents appointed to produce the material 
of limestones, and also to make even the flint and many of the 
siliceous deposits of the earth’s formations. 

Coral is never, therefore, the handiwork of the many- 
armed polyps; for it is no more a result of labor than bone- 
making in ourselves. And again, it is not a collection of cells 


CORAL AND CORAL MAKERS. 19 


into which the coral animals may withdraw for concealment 
any more than the skeleton of a dog is its house or cell; for 
every part of the coral—or corallum as it is now called in sci- 
ence—of a polyp, in most reef-making species, is enclosed 
more or less completely within the polyp, where it was 
formed by the secreting process. 

It is not, perhaps, within the sphere of science to criticise 
the poet. Yet we may say in this place, in view of the frequent 
use of the lines even by scientific men, that more error in the 
same compass could scarcely be found than in the part of 
Montgomery’s ‘“‘ Pelican Island” relating to coral formations. 
The poetry of this excellent author is good, but the facts nearly 
all errors—if literature allows of such an incongruity. There 
is no ‘‘toil,” no “skill,” no ‘‘ dwelling,” no “ sepulchre” in the 
coral plantation any more than in a flower-garden ; and as lit- 
tle are the coral polyps shapeless worms that “writhe and 
shrink their tortuous bodies to grotesque dimensions.” 

The poet oversteps his license, and besides devrades his 
subject, when downright false to nature. 


Coral is made by organisms of four very different kinds. 
These are: st, Potyps, the most important of coral-making 
animals, the principal source of the coral reefs of the world. 

Second, Animals related to the littie Hydra of fresh waters, 
and called Hyprorps (a division under the Acalephs), which, 
as Agassiz has shown, form the very common and often large 
corals called Millepores. 

Third, The lowest tribe of Mollusks, called Bryozoans, 
which produce delicate corals, sometimes branching and moss- 
like (whence the name from the Greek for moss animal), and at 
other times in broad plates, thick masses, and thin incrusta- 
tions. Although of small importance as reef-makers at the 


20 CORALS AND CORAL ISLANDS. 


present time, in a former age of the world—the Paleozoic— 
they so abounded over the sea bottom that some beds of lime- 
stone are half composed of them. 

Fourth, Alge or sea-weeds, some kinds of which would 
hardly be distinguished from corals, except that they have no 
cells or pores. 


fT POLYES: 


A. good idea of a polyp may be had from comparison with 
the garden aster; for the likeness to many of them in external 
form as well as delicacy of coloring is singularly close. The 
aster consists of a tinted disk bordered with one or more series 
of petals. And, in exact analogy, the polyp flower, in its 
most common form, has a disk fringed around with petal-like 
organs called tentacles. Below the disk, in contrast with the 
slender pedicel in the ordinary plant, there is a stout cylindri- 
cal pedicel or body, often as broad as the disk itself, and some- 
times not much longer, which contains the stomach and inter- 
nal cavity of the polyp; and the mouth, which opens into the 
stomach, is at the centre of the disk. Here then the flower- 
animal and the garden-flower diverge in character, the dif 
ference being required by the different modes of nutrition and 
other characteristics in the two kingdoms of nature. ‘The cor- 
al polyp is as much an animal as a cat or a dog. 

The figures of the frontispiece, and others on pages 23, 24, 
26, sustain well the description here given, and afford some 
idea also of the diversity of form among them. 

The prominent subdivisions of polyps here recognized are 
the following: ) 

I. Acrinom Potyps.—Related to the Actinia, or Sea-anem- 


one, in tentacles and interior structure, and having, as in 


PLATE 11. 





1.Phymactis veratra ; 2, P. clematis. 





CORAL AND CORAL MAKERS. 21 


them, the number of tentacles and interior septa a multiple of 
six. The name Actinza is from the Greek for ray. 

II. CyatuopHyLioip Potyps.—Nike the Actinoids in tenta- 
cles and interior structure, except that the number of tentacles 
and interior septa is a multiple of four. Ludwig and De 
Pourtales state that the number in the earliest young state is 
siz, and that therefore the fundamental ratio is the same as in 
the Actinoids; and that they pass from this ratio by develop- 
ments of tentacles and septa more rapidly on one side than the 
opposite, and in such a manner that the number becomes after 
the first stage a multiple of four. The Cyathophylloid polyps 
hence combine this characteristic of the Actinoids with one 
feature of the Alcyonoids. The Cyathophylloids were the ear- 
liest of polyps, and the most abundant species in Paleozoic 
time. 

III. Aucyonorw Potyrs.—Having eight fringed tentacles, 
and other characters mentioned beyond; as the Gorgoniz and 


Alcyonia. 


I -ACTINOID: POLYPS: 


The highest of Actinoid Polyps are those of the Actinra 
TRIBE—the species that secrete no coral to clog vital action 
and.prevent all locomotion. ‘The details of structure may be 
best described from the Actinia or Sea-anemone, and after-— 
ward the distinguishing characters of the coral-making polyps 
may be mentioned. In external aspect and in internal charac- 
ters all are essentially identical. 


1. NON-CORAL-MAKING POLYPS. 


As the colored figures on Plate II., and also the following. 


show, the external parts of an Actinia are —a subcylindrical 


22 CORALS AND CORAL ISLANDS. 


body—a disk at top—one or more circular series of tentacles 
making a border to the disk—a mouth, a merely fleshy, toothless 
opening, at the centre of the disk, sometimes at the summit 
of a conical prominence—a basal disk for attachment. The 
upper extremity is called the acténal end, since it bears the 
tentacles or rays, and the lower or base, the abactinal. 
Sea-anemones vary greatly in color, and in the distri- 
bution of their tints. This is finely illustrated on the first 
four plates of the Author’s Atlas of Zodphytes. Two figures 
of Plate I. are reproduced on the accompanying Plate IL: 
one, Phymactis clematis, from Valparaiso, and the other, Phy- 
mactis veratra, from Wollongong, New South Wales. An- 
other variety of P. clematis has a pink disk, wine-red tenta- 
cles, and the body reddish with dots of dark green. The 
P. florida, from the coast of Peru, one variety of which 1s 
shown on the second plate of the Atlas, has blue tentacles 
and a paler disk; another has a bluish green disk with pur- 
plish tentacles and the papillee of the body dark sap-green on 
a pale reddish ground; and another is green throughout. 
While often brilliantly colored, especially in the tropics, 
other Actiniz are nearly colorless. This was the case with 
that represented in the following cut, a species from Long 
Island Sound near the New Haven Light-house, figured 
some twenty years since by the author, but left undescribed. 
The body in this species had a delicate texture throughout, its 
walls being so transparent that the crgans within could be 
seen through them. It was exceedingly flexible and passed 
through various shapes, imitating vases of many forms, wine 
glasses, goblets, etc. It was generally very slow in its 
changes, and sometimes continued in the same vase-attitude 


for a whole day. 
Actinie vary immensly in size,—from an eighth of an inch 


ACTINIA AND OTHER ACTINOID POLYPS. 25 


and smaller in the diameter of the disk to over a fout,— 
though commonly between half an inch and three inches. 
One species from the Paumotu Coral Archipelago in the 





PARACTIS RAPIFORMIS, EDW. 


Pacific, a colored figure of which is given in the Atlas of the 
Author’s Report on Zodphytes (Plate III.), had a diameter 
across its disk of fowsteen inches; and it was also one of the 
most beautiful in those seas, having multitudes of tentacles 
with carmine tips and yellowish bases, around the open centre, 
gathered into a number of large groups or lobes. 

With rare exceptions, Actiniz live attached to stones, 
shells, or the sea bottom, or are buried at. base in the sand or 
mud. The attached species have the power of locomotion, 


through the muscles of the base, but only with extreme slow- 


24 CORALS AND CORAL ISLANDS. 


ness. The loose stones on a sea-shore near low tide level 
often have Actiniz fixed to their under surface. A very few 
species swim or float at large in the ocean. 

Now and then an Actinia puts itself on the back of a 
crab, and thus secures rapid locomotion, but only at the will 
of the crab, which inay at times give it some hard rubs:—a 





CANCRISOCIA EXPANSA ST., ON THE BACK OF DORIPPE FACCHINO. 


kind of association styled conumensalism by Van Beneden, as 
the two in a sense live at the same table, without preying 
one upon the other. In the above example, from the China 
seas, the Actinia has mounted a Dorippe. The figure is from 
the Proceedings of the Essex Institute, where an account of it 
is published by Prof. Verrill; the specimen was collected by 
the zodlogist, Dr. W. Stimpson. As Prof: Verrill states, the 
Dorippe carries, for its protection when young, a small shell over 
its back, which it holds in this position by means of its two 
reversed pairs of hind legs. The Actinia appears to have fixed 
itself, when young, to the shell, and afterward, by its growth, 
spread over the back of the crab, taking the place of the shell. 

This case of commensalism, like most others, is not a mere 


chance association of species; for the two always go together, 


ACTINILA AND OTHER ACTINOID POLYPS. 25 


the Actinia, according to Dr. Stimpson, never being seen 
except upon the crab’s back, and the crab never without its 
Actinia. The fact shows an instinctive liking on the part of 
the Actinia for a Dorippe courser, and for the roving life 
thus afforded it. And the crab is undoubtedly conscious that 
he is carrying his fortress about with him. It is not a soli- 
tary case; for there are many others of Actiniz attaching 
themselves to locomotives—to the claws or backs of crabs, or 
to shells in possession of soldier crabs, or to a Medusa; and 
frequently each Actinia has its special favorite, proving an 
inherited instinctive preference for rapid change of place, and 
for just that kind of change, or range of conditions, which the 
preferred commensal provides. Prof. Verrill has an interest- 
ing article on this subject, with especial reference to crustace- 
ans, in the third volume of the American Naturalist. 

Species living in sand are often unattached; and then the 


g, and sometimes balloon-shaped ; 


base is rounded or tapering, 


some of them are long and almost worm-like, and even burrow 
hke worms. | 

The following are figures of three species: one, figure 3, 
exhibiting simply the tentacles and disk of the Actinia, the 
only parts visible above the sand; the others showing the 
whole body removed from the sand, and consequently a little 
out of shape. They are from Gosse’s “ British Sea-Anem- 
ones,” in which they are given with the natural colors. 
Figure 1 represents the Peachia hastata of Gosse, a beautiful 
species having twelve large tentacles; fig. 2, the Adwardsia 
callimorpha.G.; fig. 83, Halocampa chrysanthellum G. Most 
of these sand-dwellers bury themselves like the Halocampa, 
and often hide all the disk but the mouth. The Edwardsia 
is peculiar in having, above the hollow bladder-like basal 
portion, a firm opaque exterior to the body, making for it 


26 CORALS AND CORAL ISLANDS. 


a kind of case or jacket, into which the upper extremity, 
which is soft and delicate in texture, may be retracted. The 
thickening of the epidermis in this middle portion is produced 


through the entangling of disintegrated cells and minute for. 





1. PEACHIA HASTATA, G.; 2. EDWARDSIA CALLIMORPHA, G.; 3. HALOCAMPA 


CHRYSANTHELLUM, G. 


eign particles, sometimes in part spores of Confervx, by 
means of the mucus of the surtace; and if the layer is re- 
moved, as it may be, the skin will again become covered. 
This species, like others of the genus, lives buried to its neck 
in the sand, that is, with the soft upper extremity protrud- 
ing If disturbed, the head is suddenly drawn in, together 
with more or less of the following jacketed part of the body. 

The warty prominences on some warty species have the 
power of clinging by suction to a surface, and such Actiniz 
often cover their sides thus with bits of shell or of other sub- 


stances at hand. Where there are no warts the contracted 


ACTINIAA AND OTHER ACTINOID POLYPS. rie | 


exterior skin, reticularly corrugated, occasionally becomes a 
surface of suction-warts, as in many Sagartie. 

The znternal structure of the Actinia is radiate like the ex- 
ternal, and more profoundly and constantly so. The mouth, 
a fleshy toothless opening in the disk, opens directly into a 
stomach, which descends usually about a third of the way to 
the base of the body; its sides are closed together unless 
it be in use. The general cavity of the body around and be- 
low the stomach is divided radiately by fleshy partitions, or 
septa, into narrow compartments; the larger of these septa 





connect the stomach to the sides of the animal, and, besides 
holding it in place, serve to pull it open or distend it for the 
reception of food. The above figure represents in a gener- 
al way a horizontal section of the body through the stomach, 
and shows the position of the radiating septa and the interme- 
diate compartments. It presents to view the fact that these 
are in pairs, and another fact that the number of pairs of par- 
titions in the ordinary Actinoid polyps is regularly some mul- 
tiple of six, although other numbers occur during the succes- 
sive developments that take place in the growth of a polyp, 
and are occasionally persistent in the adult state. There are six 
pairs in the first series; s¢z in the second; twelve in the third; 
twenty-four in the fourth; forty-erght in the fifth, and so on. 


28 CORALS AND CORAL ISLANDS. 


The compartment between the two septa of each pair opens 
at top into the interior of a tentacle, and thus the cavity in 
each tentacle has its special corresponding compartment below. 
This tentacular compartment is properly, as first recognized by 
Prof. Verrill, the a@mbulacral, since each corresponds in posi- 
tion and function to an ambulacral or tentacle-bearing section 
in the Echinoderms and other Radiate animals. 

Although polyps are true Radiates, they have something 
of the antero-posterior (or head-and-tail) polarity, with also the 
right-and-left, which is eminently characteristic of the animal 
type. This is manifested in the occurrence in some polyps of 
aray on the disk different in color from the general surface: 
of one tentacle larger than the others, and sometimes peculiar 
in color; of two opposite septa in a calicle or polyp-cell larger 
than the others, and sometimes meeting so as to divide the cell 
into halves. ‘The first of these marks the author has observed 
in a Zoanthid, as mentioned in his Report on Zodphytes at 
page 419, and represented on plate 30: and the last is very 
strongly developed in the cells of many Pocillopore (ib. p. 523). 
Gosse and many other authors have drawn attention to the 
one large tentacle, and the fact that it lies in the direction of 
the line of the mouth. Prof. H. James Clark, in his Mind in 
Nature, states that the order in which the fleshy septa and the 
tentacles in an Actinia are developed has direct reference to the 
right and left sides of the body, and that there is only one 
plane in which the body can be divided into two halves, and 
this is that corresponding with the longer diameter of the stom- 
ach. or the direction of the mouth. Mr. A. Agassiz has 
shown that in Actinie of the genus Arachnactis, the new 
septa and tentacles are developed either side of the one chief 
or anterior tentacle; and Prof. Verrill, that in Zoanthids, 
they are formed principally either side of this anterior tentacle 


ACTINLH AND OTHER ACTINOID POLYPS. 29 


and also of the opposite or posterior one, and much less 
rapidly, if at all, along the sides intermediate. ‘This chief: 
tentacle marks properly the true front or anterior side of 
the polyp. A fore-and-aft structure is also very strongly 
marked in some of the ancient cyathophylloid corals, and 
hence it belonged to the type from early Paleozoic time. 

The way leading out from the Radiate structure is thus 
manifested by these tlower-like polyps. In fact perfect circu- 
lar series in organs or parts do not belong to any living organ- 
ism, not even to the true flower; for growth is fundamentally 
spiral in its progress, and there must be always an advance 
end to the spiral of growth; all apparent circles are only dis- 
guised spirals. 

The walls of the body contain two sets of muscles, a circu- 
lar and a longitudinal, the latter becoming radial in the disk 
and base. Similar muscles exist also in the tentacles, and cor- 
responding muscles in the fleshy partitions or septa of the in- 
ternal cavity. 

By means of these muscles an Actinia, whenever disturbed, 
contracts at once its body; and most species make of them- 
selves a spheroidal or conoidal lump, showing neither disk 
nor tentacles. One example of this contracted state is presented 
on the frontispiece in figure 3a. After a brief period of quiet 
the polyp commonly reassumes its full expansion. The ex- 
pansion depends on an injection of the structure with salt wa- 
ter, which is taken in mainly by the mouth. As the whole body 
is thus filled and injected, the flower slowly opens out, and 
shows its petal-like tentacles. On contraction the water is 
suddenly expelled through the mouth, and by pores in the sides 
of the polyps, and at the extremity of the tentacles, and the 
tentacles disappear, along with the disk, beneath the adjoining 
sides of the body which are drawn or rolled in over them. 


30 CORALS AND CORAL ISLANDS. 


The Actinia appears, at first thought, to be well prepared 
for securing its prey through its numerous tentacles. But 
these are generally too short for prehension. Yet the disk often 
aids them by rolling over the captured animal, and pushing it 
down into the stomach. At the same time, the mouth and 
stomach are both very extensile, so that an Actinia may swal- 
low an animal nearly as large as itself; it gradually stretches 
the margins of the mouth over the mollusk or crab, until the 
whole is enclosed and passed into the digestive sac; and when 
digestion is complete, the shell and any other refuse matters 
are easily got rid of by reversing the process. 

But the Actinia owes nearly all its power of attack to its 
concealed weapons, which are carried by myriads. These 
are what Agassiz has called dasso-cells, because the little cell- 
shaped sheath contains a very long slender tubular thread 
coiled up, which can be darted out instantly when needed. 
As first observed by Agassiz, the tubular lasso escapes from 
the cell by turning itself inside out, the extremity showing it- 
self last, and this is usually done “ with lightning-like rapidi- 
ty.” Then follows the poison. The lasso-cells (called often 
nettling cells, and by Gosse cnide, and thread capsules) are 
usually less than a two-hundredth of an inch in length; but 
they are thickly crowded in the larger part of the skin or walls 
of the tentacles, and about the mouth; also in the walls of the 
stomach, and within the visceral cavity in white cords hanging 
in folds from the edge of the septa. Thus the polyp is armed 
inside and out. The mollusk or crab that has the ill luck to 
tall, or be thrown by the waves, on the surface of the pretty 
flower is at once pierced and poisoned by the minute lassos, 
and is rendered incapable of resistance. 

The following figures, by Dr. Karl Mobius, of Hamburg, il- 
lustrate admirably these organs. The views are magnified 


Plate Ill. 


Lear 
SS, 


er ™, . 
ii. 
© Srey 


: ¥ : 
= . * “ 
pie ar, aa Eh Ry, Pant "ane “alas ean eeewe Aap sbbegyetety taa tk tly DE ene t Cease Prue CUT PrCEUNTLETTUVOTe VY RYU DTS TRICE EN TDEVET TYRESE TTRTTT| ol 
" 5 , ~ * ‘ ~ se * — Pa pe Gala Pa ee i lee 
ee a tec ees ea nD Selig eae aaa aes, SR =e Seale, mig a mies Be 


“ : ere 
Sages ? . 
a ein es Se Chi RAT 


a 





LASSO-CELLS, 


‘HARVARD UNIVERSHY 
CAMBRIDGE. MA USA 


“mice WBRARY 


bet 


t 


* 
i* . 





ACTINLGi AND OTHER ACTINOID POLYPS. oD 


700 diameters. Figure 1 represents one of the lasso-cells of 
the Actinia, Corynactis viridis, with its lasso coiled up within, 
its actual length is about a 350th of an inch. Figure 2 is the 
same with the lasso out, though less than half of the long 
thread is shown. Figure 3 is the lasso-cell of the polyps of a 
European coral, the Caryophyllia Smithii. Jt differs from 
figure | in having the basal part of the lasso within the cell or 
sheath strait and stout; it is this part which makes the first 
portion of the extended lasso. <A view of part of the latter is 
represented in figure 4, and of the extremity of the same in 
figure 5. The lasso-cells in the above species are from a 240th 
to a 360th of an inch in length. In the Metridium margina- 
tum, an American Actinia occurring along the coast of the Uni- 
ted States, north of New York, the length of one of the lasso- 
cells, according to Dr. Leidy, was about a 400th of an inch, 
and the character of the extended lasso was much like that of 
figure 4. The lower part of the lasso, for a length 14 times 
or more longer than the cell or sheath, is usually thickened, 
and sometimes slenderly spindle-shaped, while the rest is an 
_ even slender thread; and the thickened part and sometimes 
all the rest, as above shown, is spirally wound by a slender 
line, sometimes elevated, set with short hairs or bristles. The 
thread-like portion nay be wanting or very short. The lasso 
is often twenty times as long as the cell or sheath, and occa- 
sionally forty times; but if the thread-like part is absent, only 
one and a half to two times. 

A lasso-cell once used is afterward worthless; for the tube 
cannot be returned to the sheath. But those thus expended 
are not missed, as the polyp has indefinite supplies of such 
weapons, and also ready means of refurnishing itself. 

Figures 6, 7, 8, 9, 10, on the preceding page, illustrate 


different stages in the development of a lasso-cell (fig. 10) 
3 


34 CORALS AND CORAL ISLANDS. 


out of a common spherical cell, as ade out by Dr. Mobius in 
his careful microscopic investigations. The Actinia affording 
the results was the Urticina crassicornis, found in both Euro- 
pean and American seas. The actual size of the cell represent- 
ed in figure 6 is about a 5,000th of an inch. In fig. 7 the 
lasso-cell has already taken form but is folded on itself; in 8, 
there is a second infolding; 9 shows a return to a single fold, 
and further progress in the forming cell; and 10, the straight- 
ened lasso-cell. Thus the work of replenishing, throughout 
the body wherever lassos are used, is always going on. 

The radiating partitions or septa in the internal cavity 
of the polyp have along the outer free edge what looks like a 
slender white cord attached to it by a much convoluted or 
mesentery-like membrane; and this cord contains vast num- 
bers of lasso-cells radiately arranged. These white cords 
through the multiplied plaitings of the mesenteric membrane 
have great length; and they sometimes extend up through the 
stomach and pass out of the mouth; or they are extended in 
loops through the walls wherever they may happen to be torn. 

There are often also bunches of somewhat similar white cords 
full of lasso-cells appended to the septa, which are extended 
from the body through some natural orifices near the base of the 
Actinia (especially those of the Sargartia family). Gosse calls 
these cords Acontia. ‘They extend out usually two or three 
inches, and sometimes six inches, and thereby widen much the 
stinging range of an Actinia, both for the purposes of defence 
and attack. 


Gosse, in his “‘ British Sea-Anemones,” 


gives the results of 
some experiments with regard to the action of these lasso-cells 
(cnide), from which a few paragraphs may be here cited. 

“Tt has long been known, that a very slight contact with 


the tentacles of a polyp is sufficient to produce, in any minute 


ACTINIA AND OTHER ACTINOID POLYPS. ao 


animal so touched, torpor and speedy death. Since the discoy- 
ery of these cnid@ (lasso-cells) the fatal power has been sup- 
posed to be lodged in them. Baker, a century ago, in speak- 
ing of the Hydra, suggested that “there must be something 
eminently poisonous in its grasp ;” and this suspicion received 
confirmation from the circumstance that the Hntomostraca 
which are enveloped in a shelly covering frequently escape un- 
hurt after having been seized. The stinging power possessed 
by many Meduse, which is sufficiently intense to be formida- 
able even to man, has been reasonably attributed to the same 
organs, which the microscope shows to be accumulated by mil- 
lions in their tissues. 

“Though I cannot reduce this presumption to actual cer- 
tainty, I have made some experiments, which leave no reason- 
able doubt on the subject. First—I have proved that the ecthor- 
eum (tubular thread of the lasso-cell) when shot out, has the 
power of penetrating, and does actually penetrate, the tissues 
of even higher animals. Several years ago, I was examining 
one of the purple acontia of Adamsia palliata ; no pressure 
had been used, but a considerable number of cnidw had been 
spontaneously dislodged. It happened that I had just before 
been looking at the-sucker-foot of an Asterina, which remained 
still attached to the glass of the aquatic box, by means of its 
terminal disk. ‘The cilia of the acontéwm had, in their rowing 
action, brought it into contact with the sucker, round which it 
then continued slowly to revolve. The result I presently dis- 
covered to be, that a considerable number of the cnidw@ had 
shot their ecthorea into the flesh of the sucking disk of the 
Echinoderm, and were seen sticking all round its edge, the 
wires (lassos) being embedded in its substance even up to the 
very capsules, like so many pins stuck around a toilet pin- 


cushion. 


36 CORALS AND CORAL ISLANDS. 


“To test this power of penetration still farther, as well as 
to try whether it is brought into exercise on the contact of a 
foreign body with the living Anemone, I instituted the follow- 
ing experiment. With a razor I took shavings of the cuticle 
from the callous part of my own foot. One of these shavings 
I presented to the tentacles of a fully expanded Tealia crassi- 
cornis ( Urticina crassicornis of Europe and America). After 
contact, and momentary adhesion, I withdrew the cuticle, and 
examined it under a power of 600 diameters. I found, as 1 
had expected, enide standing up endwise, the wires in every 
ease shot into the substance. They were not numerous—in a 
space of .01 inch square, I counted about a dozen. * * * 

“These examples prove that the slightest contact with the 
proper organs of the Anemone is sufficient to provoke the dis- 
charge of the cnid@; and that even the densest condition of 
the human skin offers no impediment to the penetration of the 
ecthorea. 

“ As to the injection of a poison, it is indubitable that pain, 
and in some cases death, ensues even to vertebrate animals 
from momentary contact with the capsuliferous organs of the 
Zodphyta. * * * I have elsewhere recorded an instance 
in which a little fish, swimming about in health and vigor, 
died in a few minutes with great agony through the momen- 
tary contact of its lip with one of the emitted acontia of Sa- 
gartia parasitica. It is worthy of observation, that, in this 
case, the fish carried away a portion of the acontium sticking 
to its lip; the force with which it adhered being so great, that 
the integrity of the tissues yielded first. The acontium severed, — 
rather than let go its hold. 

‘* Now, in the experiments which I have detailed above, we 
have seen that this adhesion is effected by the actual impene 
tration of the foreign body by a multitude of the ecthorea, 


ACTINL# AND OTHER ACTINOID POLYPS. 37 


a 


whose barbs resist withdrawal. So that we can with certainty 
associate the sudden and violent death of the little fish with 
the intromission of barbed ecthorea.” 

The following observation Ly J. P. Couthouy, from the 
author's Report on Zodphytes (p. 128), if it is beyond ques 
tion, shows power even in the Actinia’s presence. “ Having a 
number of Monodontas (a genus of univalve Mollusca allied 
to our Trochi) too much crowded in a large jar of water. I 
took out half-a-dozen, and placed them in a jar with an Ac- 
tinia (Anthea flagellifera).- On looking at them about three 
hours after, | found that, instead of climbing like the others 
to the top of the water, they remained just where they had 
fallen, closely withdrawn into their shells. Supposing them 
to be dead, they were taken out, when they directly began to 
emerge; and when returned to the jar with the other Mono 
dontas, they were in less than five minutes clustered round 
its mouth. On placing them again in the jar with the Ac- 
tinia, though kept there for two hours, they did not once 
show themselves out of the shell. Once more placing them 
along with the other shells, they exhibited their former signs of 
life and activity. The experiment was repeated several times 
with a large Littorina, with the same result, evincing fear of 
the Actinia on the part of the Mollusks.~ 

Gosse states the following fish story, which is much to the 
point. Speaking of the Anthea cereus, or Opelet, a British 
species, he says (p. 168): “I one day saw an amusing example 
of its power of passive resistance. A beautiiul little speci- 
men of the variety alabastrina, which had been sent to me 
by Mr. Gatehouse, I had occasion to remove from one tank 
to another. There was a hali-grown Bullhead (Cottus bubalis) 
at the bottom, which had been in captivity rather more than 

a fortnight. As he had not been fed during that time, I pre 


38 CORALS AND CORAL ISLANDS, 


sume he was somewhat sharp-set. He marked the Anthea 
falling, 
ern of a mouth and sucked in the bonne bouche. It was not 


and before it could reach the bottom, opened his cay- 


to his taste, however, for as instantly he shot it out again. Not 
discouraged, he returned to the attack, and once more sucked 
it in, but with no better success; for, after a moment’s rolling 
of the morsel around his mouth, out it shot once more; and 
now the Bullhead, acknowledging his master, turned tail, and 
darted into a hole on the opposite side of the tank in manifest 
discomfiture.” 

He adds: ‘‘ But if you, my gentle reader, be disposed for 
exploits in gastronomy, do not be alarmed at the Bullhead’s 
failure: only take the precaution to “‘cook your hare.” Risso 


calls this species ‘‘ edulis,” 


and says of it,—‘t On le mange en 
friture,” and I can say, “‘probatum est.” No squeamishness 
of stomach prevents our volatile friends, the French, from 
appreciating its excellence; for the dish called Aastegna, 
which is a great favorite in Provence, is mainly prepared from 
Anthea cereus. 1 would not dare to say that an Opelet is as 
good as an Omelet; but chacun a son gout—try for your- 
selves, ‘The dish is readily achieved.” 

The stomach, although without a proper sphincter muscle 
at its inner extremity, appears to be closed below during the 
process of digestion. When digestion is complete, the refuse 
from the food is pushed out through the mouth, the only ex- 
ternal opening to the alimentary cavity, and the digested ma- 
terial passes downward, into the interior cavity; and there, 
mixed with sea-water from without, it is distributed through 
all the interior cavities of the polyp for its nutrition, The 
polyp has no circulating fluid but the results of digestion 
mixed with salt water, no blood-vessels but the vacuities among 


the tissues, and no passage-way for excrements excepting the 


ACTINIG AND OTHER ACTINOID POLYPS. 39 


mouth and the pores of the body that serve for the escape of 
water on the contraction of the animal. 

Actiniz have usually no gills or branchie for the aeration 
of the blood, the whole surface of the body being ordinarily 
sufficiently soft and delicate to serve in this function. Some spe- 
cies live half buried in the sand, and, as this in large species 
would prevent the skin of the sides from aiding in respiration, 
there are sometimes very much lobed and crimpled organs, 
attached to, or alongside of, the tentacles, which give the an- 
imal-flower much greater beauty, and at the same time increase 
the extent of surface for the purposes of eration; they are 
set down as branchial by Prof. Verrill. 

In one tribe of polyps closely related to the Actinic, the 
Zoanthids, in which the outer skin is usually somewhat cor- 
riaceous, or is filled with grains of sand, there are narrow gills 
arranged vertically, one either side of the larger radiating sep- 
ta, figures of which are given in the author’s Zodphyte Atlas. 

As to senses, Actiniz, or the best of them, are not quite 
as low as was once supposed. For, besides the general sense 
of feeling, some of them have a series of eyes, placed like a neck- 
lace around the body, just outside of the tentacles. The yel- 
low prominences in this position on the larger figures in the 
frontispiece are these eyes. They have crystalline lenses, and 
a short optic nerve. Yet Actinie are not known to have a 
proper nervous system: their optic nerves, where they exist, 
are apparently isolated, and not connected with a nervous 
ring such as exists in the higher Radiate animals. 

Reproduction is carried forward both by ova and by buds, 
though the latter method is mostly confined to the coral-mak:- 
ing polyps. 

The ovarian and spermatic functions belong to the radia- 
ting septa in the interior cavity of the Actinia, and to the part 


40 CORALS AND CORAL ISLANDS. 


of a septum, mesenteric in character, at or near the outer mar- 
gin. They have the aspect of a pulpy mass, or look like clus- 
ters of ovules. The ova have no chance for escape except 
through the stomach and mouth. They are covered with vi- 
bratile cilia, and rove about free for a while. As the develop- 
ment of the embryo goes forward, a depression begins at one 
end, which deepens and becomes a stomach, with the entrance 
to itas a mouth. Concurrently, septa grow out from the inner 
wall, and a few tentacles commence to rise around the mouth. 
Not unfrequently, the young has already some of its tentacles 
before it leaves the parent. There is at first but a single row 
of tentacles; the number increases with the size until the full 
adult limit is reached, the newer series being successively the 
outer. 

In the budding process, which is of rare occurrence, Acti- 
niz grow young ones on their sides near the margin of the 
base. A protuberance begins to rise and soon shows a mouth, 
and then becomes surrounded by tentacles; and, thus begun, 
the new Actinia continues to grow, usually until its tentacles 
have doubled their number, when finally it separates from the 
parent, an independent animal. At times, as Prof. H. James 
Clark has observed, small pieces of the base of an Actinia sep- 
arate by a natural process before a trace of a tentacle has ap- 
peared, and in this case ‘‘ they do not at first show any signs 
of activity, but on the contrary, remain for a long time in a 
quiet state, having the appearance of artificially separated 
pieces, seeming to be undergoing, as in the latter, a recupera- 
tive process after the shock of a separation.” After a while 
they commence to develop and grow into perfect individuals. 
Prof. Verrill mentions the case of an Actinia from Puget’s 
Sound (the Hpzactis prolifera, V.) which had three rows of 
young individuals attached to it around the middle of its 


CORAL-MAKING ACTINOID POLYPS. 4] 


body; but whether the young Actiniz were produced by bud- 
ding from this part of the body, or whether they had colonized 
there after being produced in the ordinary way, he was un- 
able to determine. In all cases the young ultimately sepa- 
rate from the parent. 

These polyps have also the faculty of reproducing lost 
parts; and to such an extent that a mere fragment, if it be 
from the lower part and include a portion of the base, will re- 
produce all the rest of the Actinia, even to the disk, tentacles 
and stomach. Thus the mere forcible tearing of an Actinia 
from the rock to which it is attached may result in starting a 
crop of new Actiniz. 

Although Actiniz have no internal coral secretions, they 
sometimes make a thickened epidermic plate at the base, and 
also in a few cases around a part of the body. ‘This is how- 
ever not a result simply of an epidermic secretion, but arises 
from an exudation of mucus from the surface, and the entan- 
gling thereby of minute particles of foreign or dead matters. 
A case of the kind, in an Edwardsia where the body is thus 
encased, is mentioned and explained on page 25. 

The above are the more prominent characters of the Actin- 
ia tribe of polyps. The special features distinguishing them 
from the coral-making polyps are the following: (1), They are 
simple animals, or, if they bud, the buds early separate from 
the parent ; (2), They have a muscular base ; (3), They are gen- 
erally capable, more or less perfectly, of locomotion on the base 
by means of its muscles ; (4), They sometimes possess rudimen- 
tary eyes; (5), They have no internal coral secretions. Each 
of these characters is evidence of the superior grade of this di- 
vision of Polyps. 


42 CORALS AND CORAL ISLANDS. 


Il. CORAL-MAKING ACTINOID POLYPS, 


Of the form, tentacles, mouth, stomach, fleshy septa, lasso. 
cells, food, digestion and respiration of the coral-making polyps 
here included, nothing need here be said, these characters being 
the same as in the Actinie. Their more striking peculiarities 
depend on the secretion of coral, making them fixed species, 
and involving an absence of the base; and, in the case of the 
majority of the species, on the extent to which they multiply 
by buds, in imitation of species in the vegetable kingdom. 

The coral skeleton which the secretions of polyps form is 
called the corallum. 'These secretions take place among the tis- 







































































CARYOPHYLLIA CYATHUS. 


sues of the sides and lower part of the polyp, but never in the 
disk or stomach, as this would interfere with the functions of 
these organs. In the above sketches of a simple coral, 
from the Mediterranean, the upper extremity is a depression, 


or calicle, enclosed by a series of radiating calcareous (coral) 


CORAL-MAKING POLYPS. 43 


septa. Each of these septa is secreted between a pair of the 
radiating fleshy partitions, or septa, of the polyp (see figure 
p. 27); and thus the radiate structure of ordinary corals is 
nothing but an expression of the internally radiate structure 
of the polyp. When alive, the top, and usually the sides, of 
the coral were concealed by the outer skin of the polyp, in- 
cluding, above, the disk and tentacles; and into the depression 
or calicle at top, descended the stomach. 

Whether these radiating septa of the coral are secreted 
from the surfaces of the fleshy septa, or from a prolongation 
inward of the membrane forming the walls of the internal 
cavity, has not been directly ascertained. The latter view is 
sustained by Prof. Verrill, on the ground that the coral 
septa contain fibres of animal tissue. The secretion does not 
always commence at the central plane of a septum, for the 
septa are sometimes hollow within, just as the surface spines 
of some species (@. g., Hchinopora reflea) are hollow. The 





THECOCYATHUS CYLINDRACEUS, Pout: ; FLABELLUM SPHENISCUS, 1Dy 


exterior surface of the corallum, that is, the part outside of 
the calicles, is often ribbed, and the ribs are ordinarily only 
an outer extension of the interior septa; so that surface 
spines are in fact but the outer margins of septa. 

The first of the preceding figures exhibits another of the , 


forms of these simple corals. It is described by Pourtales 


44 CORALS AND CORAL ISLANDS. 


from specimens collected by him at a depth of 100 to 200 
fathoms off the Florida reef. The actual size was one-third 
that of the figure. The second figure represents a living 
species. 

The bottom of the calicle, or polyp-cell, in the corallum is 
sometimes made simply by the meeting of the radiating sep- 
ta; occasionally by the same, with the addition of a point 
or columella at the centre; often by a twisting together of 
this part of the radiated septa. Very often, also, it is a mere 
porous mass. Sometimes there is a circle of prominent points 
about the centre, as seen in the figure of a Caryophyllia on 
page 42, which are the extremities of narrow vertical strips 
(called palz) lying in the planes of the septa. Similar points 
exist in the Thecocyathus on the preceding page, though not 
in sight in the figure. 

in many cases the bottom is quite solid; and this may be 
so either (1), because the coral secretions fill up all the pores 
as the polyp increases in age, and thus make the inte- 
rior of the corallum solid or nearly so; or (2), because there are 
formed periodically, as the polyp grows upward, solid horizon- 
tal plates across the bottom, so that beneath, in the interior of 
the corallum, there is a series of plates or tables with spaces 
between. The Pocilloporze, among recent corals (p. 70), and the 
Favosites among ancient, are examples. Increasing solidity 
with the increasing age of the polyps is also produced at 
times by additions to the exterior ef a corallum. In many 
species, the skin, over part or all of the exterior, gradually 
disappears or dies away and leaves the corallum bare, while 
all is living within; and, in such cases, the skin, before disap 
pearing, often adds a layer of stony material to the exterior, 
viving greater firmness to the whole. An example is shown 


in the figure on p. 42. In such a case, there is no skin or 


CORAL-MAKING POLYPS. 45 


aninal tissue over the outside of the corallum, excepting at 
its upper extremity, above this calcareous coating, 

Another form of a corallum, the secretion of a single 
polyp, is illustrated in the following figure of a species of the 
Fungia family, so-called in allusion to a resemblance to the 
mushroom. ‘The long mouth occupied a considerable part of 


the longitudinal central line. From the line at the centre, 












































CTENACTIS ECHINATA, AG. 


there is the same radiated arrangement of calcareous septa 
as in the preceding species, though the animal differs greatly 
in its extreme shortness in proportion to the breadth. The 
corals of this group are also peculiar in having the radiated 
upper surface flat, or nearly so, instead of concave. The fig- 
ure is a fourth the natural size. These corals, of the genus 
Fungia, often exceed a foot in length; and thus coral animals 
are sometimes as large as the largest of Actinie. 

Another species of this genus, the Hungia lacera V. (for- 
merly Pungia echinata D., trom the Feejees), is represented as it 
appears when living (excepting a part left off to suit the page) 
in the following figure. The coral in the perfect state of the an- 
imal, is wholly concealed, though often showing the points of the 
teeth of the septa in consequence of the skin being broken. 


46 CORALS AND CORAL ISLANDS. 


An enlarged view of one of the tentacles is given on the 


























































































































FUNGIA LACERA, D: (V.) 


opposite page. They are very small, compared with the size of 


CORAL-MAKING POLYPS. 47 


the polyp; and this is true of all the living Fungie studied 
by the author. It is plain that the power of such tentacles 
must reside wholly in their lasso-cells. 


~ vy 2H pag vi t 
“ol WE wy’ hi Wi WY a4 


aga Lt 





TENTACLE OF FUNGIA LACERA, ENLARGED. 


The tentacles are scattered over the disk. instead of being 
in regular circles. Nevertheless, there is a regular order of 
development, as stated on page 27; a fact which shows that 
the apparent irregularity is a consequence of the unusual 
size of the polyp and the consequent larger number of the 
radiating lamellae and polyps. 

The Fungie, unlike most corals, are not fixed animals 
except in the young state. They are common in coral-reef 
seas, lying over the sandy or rocky bottom between the other 
corals. 

Other varieties of corals and coral animals are illustrated 
in the figures on the following pages. They represent com- 


pound groups, in which great numbers of polyps are con- 





nected in a single zobphyte—a result, in part, of the process 
of budding already alluded to, and partly of different modes 
of growth connected therewith. 

This budding is very similar to the budding process in 
vegetation. One common method is the same that is occa- 
sionally met with in Actinixw, the description of which is 


briefly given on page 40. The bud commences as a slight 


48 CORALS AND CORAL ISLANDS. 


prominence on the side of the parent. ‘The prominence en- 
larges, a mouth opens, a circle of tentacles grows out around 
it, and increase continues till the young finally equals the 
parent in size. Since in these species the young does not 
separate from the parent, this budding produces a compound 
group; and the process often continues until in some instances 
thousands, or hundreds of thousands, have proceeded from a 
single germ, and the colony has increased to a large size, 
sometimes many feet, or even yards, in breadth or height 
Such is the species of Dendrophyllia represented in the fig- 
ure on page 51, and the Madrepora figured on page 50; in 
both of which, and in all such coral zodphytes, each stellate 
cavity or prominence over the surface corresponds to a sepa- 
rate one of the united polyps. 

The compound mass produced by budding—which con- 
sists of the united polyps with the corallum.as their united 
secretion—was called in the Author’s Report, a Zodphyte, it 
being truly animal in nature, though under a plant-like form 
through the plant-like process of budding. But the word to 
many minds conveys the idea that the species is something 
between a plant and an animal, which is totally false; and 
besides, it is often used distinctively for the division of ani- 
mals including the sponges. As a substitute the term Zod- 
thome may be employed, derived from the Greek gwov, ani- 
mal, and Owuoc, a heap—a term applicable also to compound 
groups in other classes, as, for example, those of Rhizopods, 
Bryozoans and Ascidians. The term zoOphyte, where employ- 
ed beyond, signifies a zoéthome formed of united polyps, or a 
polyp-zoothome. The coral of the zodthome being the coral- 
lwm, that of each polyp in the compound corallum may be 
called a corallet—the term calicle, formerly used by the 


author for the same, being now restricted to the polyp-cell. 


CORAL-MAKING POLYPS. 49 


It is obvious that the connection of the polyps in all com- 
pound groups must be of the most intimate kind. The sever- 
al polyps have separate mouths and tentacles, and separate 
stomachs; but beyond this there is no individual property. 
They coalesce, or are one, by intervening tissues; and there is 
a free circulation of fluids through the many pores or lacunes. 
The zodthome is like a living sheet of animal matter, fed and 
nourished by numerous mouths and as many stomachs. 

Polyps thus clustered, constitute the greater part of the 
flowering zodphytes of coral reefs. Only a few are simple 
animals, like the Caryophyllia figured on page 42, or the 
Thecocyathus, page 43, or the Fungia, page 46. 

This kind of budding may take place from the sides of the 
polyp at different heights ; either (1), from the base, as in the 
Actinia mentioned on page 40, when it is basal ; or (2), above 
the base, when it is called lateral; or (3), at the upper mar- 
gin outside of the tentacles, when it is called marginal or sw 
perior ; or (4), from the disk inside of the tentacles. 

Sometimes a shoot grows out from one point only of the 
base of a polyp, like the stoloniferous stem from a strawberry 
plant, and at short intervals gives off buds; and thus makes a 
linear zo6phyte with a row above of flower-animals. In other 
cases, the base spreads in all directions and buds at the edge, 
or in the upper surface near the edge, and so makes an in- 
crusting plate, consisting of a multitude of polyps. 

If the germ polyp, or that from which the compound zo6- 
phyte proceeds, has the property of growing upward beyond 
the adult height—which the existence of coral renders a possi- 
bility, and even to an indefinite degree—various other forms 
may result. 

Sometimes the first polyp gives out buds from its sides, 
and continues so to do while it grows upward; and thus a 

4 


50 CORALS AND CORAL ISLANDS. 


rising stem is formed with one parent polyp at the extremity 
of the stem, and a terminal corallet to the corallum, or to each 
branch of it. This is the case in the genus Madrepora, a 





MADREPORA ASPERA, D. 


species of which is here represented. Each branch in the 
living state had at its extremity the parent polyp of the 
branch, or that whose budding made the other polyps of the 
branch. In such species, a new lateral branch is commenced 
by one, among the many polyps over the surface of a branch, 
beginning to grow and bud. Thus branch after branch is 
added, and the little tree produced. 

Another kind of coral, growing and budding in the same 
manner, is represented on page 51. It is a species of Dendro- 
phyllia, from the Feejees—a genus often presenting tree-like 
forms, as the name implies. 

In other cases, budding goes on until a cluster of some size 


VORAL-MAKING POLYPS. Dil 


is formed, and then the older or marginal polyps of the cluster 





DENDROPHYLLIA NIGRESCENS, D. 


cease budding while the rest continue the process ; in this way 


52 CORALS AND CORAL ISLANDS. 


a stem rises, with the budding cluster of polyps at its summit, 
and the more aged, or non-budding polyps, about its sides ; and 
the breadth of the stem depends on the size of the budding 
















































































































































































MS Me y 
wh NH H 
_ Ill I 


= 
i i —— 


IS! ih 
Deh 





ult 
i ] 





GONIOPORA COLUMNA, D. 


cluster. Above a case of this kind is represented, in which 


the stem is a large column. . 
The polyps, in this beautiful Pacific species, as seen, stand 


up prominently over the coral when expanded, which is due 


CORAL-MAKING ACTINOID POLYPS. 53 


to the fact that only the lower parts of the polyp secrete coral, 
as a moment's consideration will make apparent. 


In other cases, the budding cluster is small, and hence 





PORITES MORDAX, D. 


makes small branches, as in the annexed figure of a species of 
Porites, from the Feejees. The cells in this genus are very 


small and nearly or quite superficial, as the figure shows. 


54 CORALS AND CORAL ISLANDS. 


4 


New branches are made in such species by a forking of an 
old one, The budding cluster enlarges as it grows, and, when 
it is just beginning to pass the regular or normal size for the 
species, a subdivision of the budding cluster commences at the 
extremity of the branch. It is a process of spontaneous fis- 
sion of a branch or stem. In this way the forking in the coral 
of the figure on page 52 was produced, and also the branching 
in that on page 53. 

Sometimes, again, the budding cluster is a linear series ; 
and then a coral with erect, flattened or lamellar branches is 
made. 

Again, sometimes each branch of the corallum is only 
the corallet of a single polyp ; and new branches are added by 
the budding of new polyps trom its sides, each to lengthen out 


into a new branchlet. In this manner the coral here figured, 





CLADOCORA ARBUSCULA, LEs. 


a common species of the West Indies, and also that of fig. 1 on 
Plate LV., a Caulastrea, from the Feejee group, were formed. 
When the budding is not confined to any particular polyp, 
or cluster of polyps, but takes place universally through the 
growing mass, the coral formed is more or less nearly hemi- 


spherical; and often the process goes on with such extreme 


PLATE IV 


ss 





Caulastraea furcata; 2. Tridacophyllia pzonia; 
3. Mezandrina tenuis. 


CORAL-MAKING ACTINOID POLYPS. 55 


regularity that these hemispheres are perfectly symmetrical, 
even when enlarged to a diameter of ten or fifteen feet. A 
portion of the surface of one of these massive species, called 
Orbicella cavernosa, from the West Indies, is represented in 
the annexed figure. In the growth of these hemispheres, the 
enlargement takes place in the spaces between the polyps; and 





ORBICELLA CAVERNOSA. 


whenever these spaces begin to exceed the width usual to the 
species, a new mouth opens, commencing a new polyp; and 
thus the growth of the mass involves multiplication by buds. 
The small calicle near the centre of the figure is from one of 
the new interstitial buds. 

Species of Porites also grow into hemispheres and rude hil- 
lock-like forms, through the same method of budding, and 
some of the masses in the tropical Pacific have a diameter of 
even twenty feet. Myriads of living polyps are combined in a 
single such mass, for each is but a fifteenth or a twentieth of an 
inch in diameter. 

Often there is a lateral growth of the polyp and thereby of 
the zodphyte without much upward growth; and spreading 
leaves are thus made, and bowl-like shapes. Where there is 
lateral budding, the leaves have generally an edge of young 
polyps from the new buds that are there opening, as in the 


56 CORALS AND CORAL ISLANDS. 


Gemmipores, and some foliaceous Madrepores. Where there is 
superior budding, and sometimes in the case of inferior, the 
new polyps appear some distance from the edge, the growing 
margin spreading on in advance of the buds that open in it, 
as in the Echinopores, the Phyllastreea represented on Plate I. 
(frontispiece), and also in the Merulina of Plate VI., figure 1 


(facing page 82). 


Besides the method of budding explained in the above re- 
marks, there is also a kind of superior budding called sponta- 
neous fission, which consists in a spontaneous subdivision of a 
polyp, by which two are made out of one. In such cases the 
disk of the polyp has not a distinct limit of growth, as in the 
above, but tends to enlarge indefinitely ; and when there isa 
beginning of an increase beyond the proper adult size, a new 
mouth opens in the disk, a short distance from the old one, and 
at the same time its edges extend downward and make a new 
stomach beneath it; finally tentacles are developed between 
the two mouths, and then each polyp separates with its part of 
the old tentacles as illustrated in the following figure. It is not, 





SPONTANEOUS FISSION IN POLYPS. 


as is seen, a subdivision strictly into halves, as one carries off 
the old mouth and stomach. ‘The figure to the left represents 
a polyp of the Astra tribe, with already two mouths, through 
a commencement of the process of subdivision. In the next 


figure there are tentacles between the two mouths, so that each 


CORAL-MAKING AUTINOID POLYPS. ie 


mouth has its own circle; and in tlie third, the separation has 
gone so far as to complete the circles and make two independ- 
ent polyps. This dividing one’s self in two, for the sake of an 
increase of population, is the process called spontaneous fis- 
sion or fissiparity. 

This mode of budding does not belong exclusively to coral 
polyps, for it has been observed among a few Actiniz. Grosse 
describes its occurrence in a British species, the Anthea cereus, 
in which it results in two distinct animals. He says “the 
fission begins at the margin of the disk, and gradually extends 
downward until the separation is complete, when each moiety 
soon closes and forms a perfect animal.” The same author al- 
ludes to the occurrence of double-disked individuals of the gen- 
era Actinoloba, and Actinia as illustrating the process without 
a separation of the spontaneously developed pair. 

_ This spontaneous fission is the common kind of budding 
in the large Astrzea tribe. 





ASTRAA PALLIDA, D. 


The preceding figure represents a species of living coral of 
the Astrea family, from the Feejees, the Astrea pallida D. 


58 CORALS AND CORAL ISLANDS. 


which grew, and multiplied its polyps as it grew, by this meth- 
od. In such species some of the disks of the polyps will be 
found to have two mouths. This is the first step in the pro- 
cess In others, the two mouths will be found to be partly 
divided from one another by new-tormed tentacles ; and finally 
each will have its own circle complete and all else in polyp 
perfection. 

Many of the Astrea hemispheres of the Pacific, grown by 
this method, have a diameter of ten to fifteen feet. 

In other Astrzea-like species, this spontaneous fission ends 
in a complete separation of the two polyps formed ; and conse- 
quently in a forking of an old branch. The figure of a Cau- 
lastreea, on Plate IV. (figure 1), illustrates this mode of 
branching. In the left hand polyp there are already two 
mouths, and the work of subdivision is consequently begun ; 
while in those to the right, which have a single mouth, the 
subdivision has just been completed, and also the forking of 
the old branch. Thus spontaneous fission goes forward, and 
branches accordingly multiply. By this method some of the 
most magnificent clumps of coral zodphytes found in tropical 
seas have been, and are being, developed each from a single 
germ. Many of them have the perfect hemispherical sym- 
metry of the solid Astraas. 

Sometimes, when a new mouth forms in an enlarging disk, 
there is not at once a separation of the two, but the disk con- 
tinues to enlarge in one direction and another, and then an- 
other mouth opens, and so on until a string of mouths exists 
in one elongated disk; and finally, a separation occurs, but 
only to commence or carry forward another long series. In 
this way the corals with meeandrine furrows are made, some 
kinds of which are popularly called Brain coral, and pertain 
to the Meeandrina family (figure on page 65). The same may 


CORAL-MAKING ACTINOID POLYPS. 59 


take place in the ramose corals, and so make flat branches, 
each with a long sinuous line of polyp mouths at top. In 
all such species the tentacles stand in a line either side of 
the line of mouths as represented in figure 3 on Plate IV., 
between pages 54 and 55. 

By the simple methods here explained all of the various 
forms of Actinoid zodphytes have been produced ; and, equally 
so, those of the Aleyonoids described beyond. The tree, shrub, 
clusters of coral leaves, hemispheres, and coral net-work re- 
quire for the explanation of their origin only the few princi- 
ples which have been mentioned. The germ-polyp, growing 
upward and more or less outward, and budding as it grows, 
makes thus the rismg stem —that of the Madrepore or Den- 
drophyllia, with its summit polyp (figures p. 50, 51), or that of 
the Porites, with its terminal budding clusters (p. 53); or the 
rising, massive dome of the Astrea and Meeandrina (pp. 57, 
65), in case budding is symmetrical in all directions; or, if 
growth in the germ-polyp is upward exclusively, it forms a ris- 
ing stem bearing at top the single polyp that originated it, or 
crowded clusters of such stems branching variously and having 
each branch surmounted with its one polyp (figure p. 54); or, 
if there is lateral growth and but little of upward, it produces 
leaf-like forms and graceful groups or clusters of leaves, vases, 
and other shapes; or, if the germ-polyp is capable of lateral 
growth alone, the results are simple lines of polyps creep- 
ing over the supporting rock, like the creeping stolons of 
a plant, or else encrusting plates, spreading outward like a 
lichen. 


In the descriptions of corals the following terms have the 
significations annexed. Those already mentioned are here 
repeated to bring them all together. 


60 . CORALS AND CORAL ISLANDS. 


Zovthome. — The compound animal mass produced by budding. 

Corallum. — The coral either of the compound mass, or of the solitary 
polyp. 

Corallet (In Latin, corallulum). — The coral of a single polyp in a com- 
pound corallum. 

Calicle. —The polyp cell in the top of a corallet, or of a solitary 
corallum, within the walls of the cells; it is sometimes flat at top, that is, 
without the usual depression. 

Septa. — The radiated plates of the cell or calicle. 

Dissepiments. — Small cross plates between adjoining septa, approxi- 
mately horizontal; sometimes wanting. 

Synapticule.— Calcareous bars extending across the interseptal spaces, 
or loculi, and so uniting the surfaces of adjoining septa. 

Tabule. — Horizontal plates dividing the interior of a cell into a 
series of chambers, as in the ancient Favosites, and in the Pocilliporee. 

Tabulate. — Waving tabule. The Tabulate include the Favosites and 
some other ancient corals. 

Columella. — The prominent axis of a corallet projecting at the centre 
of @ calicle. It is generally absent. 

Coste. — The vertical ridges over the exterior of some corals; they 
usually correspond to the sept, and are an external extension of them; 
but in other cases they are opposite the intermediate chambers, or are 
interseptal Joculi, as they are often called. 

Ceenenchyma.—'The common mass of the corallutm between its dif- 
ferent polyp cells or corallets, as in the Madrepore, Gemmipore, and 
Dendrophylliz. 

Epitheca.— The coral layer sometimes deposited over the exterior of 


the corallum during the life of the polyp by the outer skin before it dries, 


away, as explained on page 44. 

Peritheca. — The epitheca of a compound group or zoothome (fig. p. 71). 

Exotheca.— The portion of the corallum outside of the walls of cells 
in many coralla of the Astreea family, and some others, in which the polyps 
of the mass are properly in contact, and there is consequently no true 
coenenchyma. 

Endotheca. —'The portion of the corallum inside of the walls of the 
cell. 


We may now state briefly the characteristics of the 
grander divisions of the Actinoid polyps, several of which 
have been illustrated in the preceding figures. 


SUBDIVISIONS OF ACTINOID POLYPS. 6] 


The tribes adopted are those recognized by Prof. Vegrill, 
and have the limits he has assigned tothem. The classification 
diverges from his system in uniting the non-coral-making 
and coral-making species into one grand division, that of the 
Actinoids (on the ground of the close resemblance of the 
polyps), and also in separating from the latter the Cyatho- 
phylloid corals, for the reasons mentioned on page 21. Some 
of the figures of corals on former pages are here repeated in 
order to present together those of like relations. 


1. Species without internal Coral Secretions. AcrinarRia of 
Verrill. 


1. The Actinza tribe, or ActTINACEA, secrete no coral inter- 
nally, and moreover have a muscular base, with some degree 
of locomotion by means of it. The Actinie of the frontis- 
piece, and of pages 23, 24, 26, are examples. 

2. The Zoanthus tribe, or Zoantuacea. The species here 
included are like the Actiniz in secreting nocoral. But while 
they have a base, it is not muscular, and they are never capable 
of locomotion. The polyps have a thick or somewhat leath- 
ery exterior, and, as already observed (p. 39), have gills, or 
branchiz. Some of the species are solitary polyps; but 
generally they form compound masses or zodthomes, 
by budding; sometimes making simple lines of polyps 
over a supporting surface; at other times incrusting plates, 
or irregular masses. The following figure (from Verrill) 
represents a species found in American seas off the coast of 
New Jersey, in deep water, and also in Massachusetts Bay, 
which has a habit of fixing ona shell for its support and of 
always taking one containing a soldier crab. The shell 
finally becomes dissolved away—how, it is not known, by 


62 CORALS AND CORAL ISLANDS. 


the growing Zoanthid; but the crab holds on to its house 
although at the expense of transporting wherever it goes a 





EPIZOANTHUS AMERICANUS, V., WITH EUPAGURUS PUBESCENS, ST. 


colony of flowering polyps. The polyps are but partly ex- 
panded in figure 1, and wholly so in figure 2. 

The animals of the Zoanthus tribe have broad, radiated 
disks, with an edging of short tentacles, in one or more 
rows. Although not secreting coral, the mucus of the sur- 
face in some of the species entangles the sand that falls 
on it, and thus gives a degree of firmness to tie mass of the 
zoophyte. 

3. The Antipathus tribe, or Antrpatuacea. In this tribe 
the polyps never have locomotion, and, so far as known, al- 
ways produce compound groups by budding. These groups 
have the forms of delicate shrubs and long twigs ; and some of 
them are three feet or more in height. The branches consist of 
a horny axis, usually spiny or hispid over its surface, surrounded 
by an animal coating, which is made up of united polyps. An 
example is shown in the following figure of a living species 
from the Feejees. A view of one of the polyps, much enlarged, is 
given in the following figure. Its tentacles are closely like 
those of the Actinia. The height of the entire shrub, collected 
by the author, was three feet, and the trunk at base was 
half an inch thick. The polyps had a brownish-yellow color, 
not particularly beautiful, and the tentacles were in general, as 





SUBDIVISIONS OF ACTINOID POLYPS. 63 


in another species described by the author, rather awkwardly 


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ANTIPATHES ARBOREA, 1D}: 


The number is commonly sza ; but 


handled by the polyp. 


in one genus, Gerardia, it is as great as twenty-four. 





POLYP OF A. ARBOREA, MUCH ENLARGED, 


64 CORALS AND CORAL ISLANDS. 


2. Polyps having internal calcareous secretions. MApDREPO- 
RARIA Of VERRILL (the Cyathophylloid species excluded). 
HEXACORALLA of some authors. 


4. Astrea tribe, or ASrraacEa.—In this tribe the polyp- 
cells or calicles are distinctly lamello-radiate within, and gen- 
erally so outside. Moreover, budding is always by division 
of the disks, or spontaneous fission. The figure of the Cau- 
lastreea, on page 58, illustrates one section of this family, that 
in which each branch of the corallum is made by a single 
polyp, and branching is by furcation through spontaneous fis- 
sion. In other related genera, as Mussa, the polyps sometimes 
have a diameter of two inclies, being as large as ordinary 
Actinie. 





ASTRA PALLIDA, D. 


The Ast7@a pallida is a good representative of the massive 
Astreas. The color of the polyps in this species is quite pale, 
the disks being bluish-gray, and the tentacles whitish. In oth- 
ers, the tentacles are emerald-green, or deep purple, or of other 


shades. 


SUBDIVISIONS OF ACTINOID POLYPS. 65 


Another range of forms is represented by the following 
figure of one of the Meandrine corals, already reterred to as 
often called ‘‘ Brain coral.” In the figure, the coral is re- 
duced one-half lineally. The difference between its mode of 
formation and that of an Astraea, has been stated on page 59. 
This species is common at the Bermudas, where it grows to a 
diameter of three feet. It is also found in the West. Indies. The 
ridges in this species are double, and hence the name Diplorza, 



























































































































































































































































DIPLORIA CEREBRIFORMIS, E. anp H. &K} 


from the Greek for double. A view of part of a living speci- 
men of a related species is shown on Plate IV. A common 
large West India species of Brain coral is called Meandrina 
labyrinthica. 

Still other forms of the Astrzea tribe are foliaceous, or such 
as would result if the growing margin of an Astreea, or of a 
Meeandrina, were to spread out into folia instead of thickening 


upward in the ordinary way. The groups of gracefully cury- 


66 CORALS AND CORAL ISLANDS. 


ing leaves are sometimes very large and symmetrical, as in 
the Tridacophyllia, Plate IV., and Merulina, Plate VI. 

2. Fungia tribe, or Functacea.— The general character 
of the simple species of this tribe is mentioned on page 45, and 
the character of the living Fungia, with its tentacles, is shown 
in the figure of a Feejee species on page 46. Large, com- 
pound groups, both massive and foliaceous, are formed by 
budding, and the budding is always superior. There are 
no margins to the disk in this tribe, and in the corallum 
of the compound kinds no wall or partition between the ad- 
jacent stars, and no walls to adjoining polyps, or only im- 
perfect ones. “The polyps consequently coalesce throughout 
by their disks. The simple Fungi are attached when young, 





FUNGIA DANA, BE. & H., REDUCED TO ONE-SIXTH LINEALLY; @,), TEETH OF UPPER 
AND LOWER MARGINS OF SEPTUM, NATURAL SIZE. 


and then would hardly be distinguished from a simple or 
solitary species of the Astrea tribe. 

3. Oculina tribe, or Ocutrnacra.—These species occur 
either simple or compound, and the latter are often branched, 
massive, or encrusting, never thin, foliaceous. Budding is 
either superior, lateral, or basal ; never by spontaneous fission. 
The coralla are remarkable for the solid walls and lamelle 


of the cells; and often for having the ceenenchyma nearly or 


SUBDIVISIONS OF ACTINOID POLYPS. 67 


quite solid. ‘Transverse septa between the lamellz are some- 
times wanting. ‘The calicles are usually striated externally 
but seldom derttate. The polyps, moreover, are small; and 
very commonly they stand prominent above the corallum when 
expanded. ‘The Orbicella, figured on page 55, is an example 
of one of the massive Astrea-like forms, constituting the Or- 
bicella family, or Orbicellide, in the Oculina tribe. 

The Caryophyllia here figured is one of the solitary species 





CARYOPHYLLIA SMITHII, STOKES. 


of the tribe found in European Seas, and on the coast of 
Great Britain. The figure is from Gosse’s British Actinology. 
It also grows much longer in proportion to the breadth. The 
figure to the right is of one unexpanded. One of its lasso- 
cells, in different states, is shown in figures 3, 4, 5, on page 31. 

The corallum of a related species, Caryophyllia cyathus, 
is given on page 42. The walls and septa are remarkably 
solid. The Caryophyllia flavus has been found not only in 
the Mediterranean, but also as far north as the British Isles, 
and in the Florida Straits. 

Another example of this tribe, as defined by Prof. Verrill, 
is the species of Astrangia occurring alive along the southern 
shores of New England, and on the coast of New Jersey. 
Specimens are not uncommon in the vicinity of New Haven, 
on the rocks by the Light-House, and at other places in Long 
Island Sound, and when alive it is an exceedingly beautiful 


68 CORALS AND CORAL ISLANDS. 


object. The accompanying figures of the animal are from the 
drawings made to illustrate a yet unpublished memoir by 
Prot. Agassiz. They are copied from the “Sea-Side Studies ” 
of Mrs. Agassiz and Alexander Agassiz. In fig. c, the polyps 
are of the natural size, while fig. @ represents one of them en- 
larged. The polyps, as is observed, stand very prominent 
above the cells of the corallum, because only the bases of 
them secrete coral; and the buds, which open between the 
ealicles, are hence lateral buds; the coral has much resem- 


blance to that of an Orbicella, in which budding is margin- 


c 


~ WW (i 
\ AN 
pW 





ASTRANGIA DANA, AG. 


al. The tentacles have minute warty prominences over 
them, which are full of lasso-cells. each about a 500th of an 
inch in length, or about two-thirds larger than those of the 
white cords that edge the internal septa. ‘The corallum, 
though massive, is somewhat irregularly lobed above, and 
grows to a diameter of two or three inches. It is covered 
with stars an eighth of an inch to a sixth across (fig. 6), which 
are usually crowded together, the intervening wall being very 
thin and solid. ‘The author alluded to the crowd of stars in 
the name Pleiadia, which he proposed for the genus in his 
Report on Zocphytes (p. 722). 

The genus Cladocora, containing slenderly branching ra- 


SUBDIVISIONS OF ACTINOID POLYPS. 69 


mose zodphytes, is closely related in its polyps, according to 
Prof. Verrill, to the Astrangiw, and belongs to that family 
Its cylindrical stems are gathered into crowded clumps. The 
C. arbuscula is figured on page 54. 


SAIN 
« 


AWS 





PHYLLANGIA AMERICANA, E. & H. 


A West India species of another genus of the group, the 
Phyllangia Americana, is represented in the annexed figure. 
In the following cut, figure 1 represents the extremity of a 
branch of an Oculina, the O. varicosa, of the family Oculinide. 





CORALS OF THE OCULINA TRIBE. 


The species of this genus grow in clumps of round branches, 


and have very solid coralla, so white and firm when bleached 


70 CORALS AND CORAL ISLANDS. 


as to go by the popular name of ‘‘ white coral,” and to be some- 
times polished for beads and other such ornamental purposes. 

Figure 2 is a branch of a beautiful little coral called Sty- 
laster erubescens Pourt., and 3, a portion of the same enlarged. 
It has the firmness, and something of the habit of an Oculina, 
but is rather like a miniature Oculina, its calicles never exceed- 
ing a twentieth of an inch in breadth. The Stylasteride 
have been shown, however, to be Hydrocorailine, like the 
Millepores (page 103). 

Figure 4, in the same cut, represents a portion of a branch of 
the Stylophora Danw KE. und H. The corals of the genus are 
remarkable for their small, crowded calicles, and for the very 
distinct six-rayed star in each calicle (as shown magnified in 
figure 5), and usually have a prominent point or columella at 
the centre of the star. The polyp of a Feejee species, S. 
mordax, is represented in figure 6. The name of the family, 

tylophoride (signifying style-bearer), alludes to this colu- 
mella. The corals grow in regular hemispherical clumps con- 
sisting of flattened or rounded branches, and are sometimes a 
foot or more across. 

In another family under this tribe, the Pociliporide, very 
common in coral-reef seas, the cells of the corallum are always 
very small and crowded, as shown in figure 7. The corals are 
branching, and in Pocillipora, the surface 1s often irregular and 
warty, the little prominences, like the rest, being covered with 
polyp cells; while in Seriatopora, the branches are slender, 
even, and pointed. The corallum in both is very firm and sol- 
id. In the larger part of them the number of tentacles is only 
twelve, and formerly they were referred on this account to the 
Madrepore tribe; a few have as many as twenty-four tenta- 


cles. 
The Pocilliporze form hemispherical clumps like the Stylo- 





SUBDIVISIONS OF ACTINOID POLYPS. ral 


phore; and the branches vary from the flattened and broad 
form shown in figure 7 (which represents the upper part of 
a branch of the P. grandis D.), to irregularly cylindrical 
branches, looking rough on account of the very short branch- 
lets. The cells are usually stellate, as in figure 8, from ge 
elongata D., and often one of the septa, and sometimes two 
opposite ones, extend to a columella at the centre, as illustra- 
ted in figure 9, from P. plicata D.; dividing the cell into 
halves. The cell in the interior of the corallum is crossed by 
thin plates or tables, as shown in figure 10, and hence they 
have been called tabulate corals. Agassiz, after the discovery 
of the Hydroid character of the animals of the Millepore corals, 
whose cells also are tabulate, referred the Pocillipore to the 
same Hydroid type. But the recent study of the polyps has 
shown that they are true polyps; and Prof. Verrill remarks 
on the resemblance of the tentacles to those of the Oculine. 
‘The stellate character of the calicle also proves that the ani- 
mals must be polyps. ; 

Madrepore tribe, or Mapreporacea.—In this tribe the cor- 
alla, even to the walls of the corallets, are remarkable for be- 
ing porous, and the radiating lamelle of the polyp-cells are 
narrow, often perforated or imperfectly developed, and fre- 
quently mere points. The coralla are either branched, mas- 
sive, or foliaceous. Budding is lateral, and in the branching 
species there is either a parent polyp, as in Madrepora and 
Dendrophyllia, or a terminal budding cluster. This peculiar- 
ity has been already illustrated in the figure of Madrepora 
aspera,on page 50. On the following page there is an out- 
line sketch of another species, the Madrepora formosa D., 
common in the Feejees, and also in the Kast Indies. The two 
species here mentioned give a good idea of the ordinary char- 
acter of the Madrepore corals. One of the polyps of the Mad. 


ie CORALS AND CORAL ISLANDS. 


repora cribripora D., a species collected in the Feejees, is rep. 
resented much enlarged in the accompanying figure. The nat- 














POLYP OF M. CRIBRIPORA, D. 


ural size of the expanded polyp in this genus is generally 
from an eighth to a twelfth of an inch across the star. The 
disk of the polyp is quite small, and the number of tentacles is 
always twelve. The most common color of the polyps is green, 
while that of the general surface between is ordinarily a pale 
or a dark umber. In many species of Madrepora the branch- 
es spread out laterally from a central or lateral trunk, and co- 
alesce together into a complete net-work, having the form of a 
shallow vase; and the interior of the vase is filled with multi- 
tudes of short, cylindrical coral stems, rising from the reticula- 
ting branches, which, when alive, have literally the aspect of 
sprigs of flowers in the vase. 

In certain kinds, closely related to Madreporie, the calicles 
are reduced to points, or spiniform or angular prominences, or 
fail altogether, and there are sometimes rounded promi- 
nences between the cells; these degraded Madrepores belong 
to the genus Montipora (Manopora of the Author’s Report). 

The genus Dendrophyllia is also referred to the Madre- 
pore tribe. The budding, as already explained, is of the 
same kind as in the Madrepores. But the tentacles exceed 
twelve. One of the polyps of D. nigrescens D., enlarged, is 
shown in the figure, on page 75. This Pacific species grows to 
a height of at least three feet, and is peculiar in having a very 








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SUBDIVISIONS OF ACTINOID POL YPS. 75 


dark blackish green or almost black color, while the polyps 
have the tentacles nearly colorless, and the disk has a circle 
of emerald green around the mouth. Dendrophyllia arborea is 
the name of a common species of this genus found in deep 





POLYP OF DENDROPHYLLIA NIGRESCENS. 


water in the Mediterranean ; it is equally large with the pre- 
ceding, and somewhat similar in its mode of branching, but 
a little stouter. It has also been found in the Atlantic about 
the Azores. Another common Mediterranean species is 
the D. cormgera. It is sparingly branched, and has very 
long and stout corallets, sometimes as long and large as the 
finger. 

The genus Gemmipora contains porous corals, of foliaceous, 
bowl-like, and massive forms, covered by prominent cylindrical, 
porous calicles, and having many short tentacles to the polyps, 
usually in a single circle. 

Here belongs also the large Porites family (Poritidz), the 
corals of which are very porous, and sometimes almost spongy, 
and whose polyp-cells are exceedingly shallow, and usually only 
imperfectly radiated. 

One of the genera in this family is Alveopora. It con- 
tains the lightest of known corals, the texture being exceeding- 


76 CORALS AND CORAL ISLANDS. 


ly porous, and the walls of the cells, which are continned reg- 


Peay 
RW). 





DENDROPHYLLIA NIGRESCENS, D. 


ularly through the corallum, are like delicate lace-work. As 


stated long since by the author, “they are intermediate in char- 


SUBDIVISIONS OF ACTINOID POLYPS. oe 


acter between the Montipore and the Favosites group ”—as 
shown by the texture and the horizontal partitions across the 





ALVEOPORA VERRILLIANA, D. 


cells, giving them the “tabulate” character of the ancient 


Favosites, as represented by the author in the annexed figure 














VERTICAL SECTION OF CORALLUM, AND UPPER VIEW OF CALICLES, ENLARGED, OF 
ALVEOPORA SPONGIOSA, D. 


exhibiting a section of the corallum of a Feejee species. On 
account of this tabulate structure, the genus was referred by 
the author to the Favosites family. A related species, of un- 
known locality, has been made the type of a new genus, called 
Favositipora, by Mr. W. S. Kent, on the ground of its tabu- 
late character (Ann. Mag. Nat. Hist., 1870), thus confirming, 


though overlooking, the author’s conclusions. 


78 CORALS AND CORAL ISLANDS. 


In the genus Porites, the corals are frequently branching, as. 


in the Porites mordax D., sometimes more slenderly, but oftener 
less so, and at times massive and monticulose in form. An- 
other species of Porites is represented on the following page, 
with one of the branches fully expanded, but the others in 
outline; a polyp, much enlarged, having twelve tentacles as 





POLYP OF PORITES LEVIS. 


in the Madrepore, is shown in the following figure. |The cells 
of the corallum are superficial, and hence the name of the 
species, Porites levis. 

Another form, different in the size and character of its 
polyps, is exemplified in the genus Goniopora. In the species 
figured on p. 52, the color of the projecting polyps was lilac 
or pale purple, and the number of tentacles eighteen to twenty- 
four, yet all were in a single series. The columns grow to a 
height of two feet or more, with only the summits for two or 
three inches alive. The dead portion is usually encrusted with 
nullipores, sponges, serpule and various shells, which protect 
the very porous corallum within from wear and solution by 
the moving waters. 


3. CYATHOPHYLLOIDS, 07 RuGosa. TETRACORALLA. 


It is not necessary to dwell here at length upon the an- 
cient Cyathophylloids. The corals have a close resemblance 
to those of the Astreea tribe in general aspect, varieties of form, 
and range of size; the methods of multiplication by buds were 
the same that are now known in the Oculina tribe. Some 


Ack 
ig 


SUBDIVISIONS OF ACTINOID POLYPS. 79 


of the larger kinds of simple corals, such as those of the gen- 


era Zaphrentis and Heliophyllum, had at times a diameter of 





PORITES LEVIS, D. 


three or four inches, so that the breadth of the polyp flower 


was probably at least six inches. Hemispherical masses of 


80 CORALS AND CORAL ISLANDS. 


solid corals attained, in some species, a diameter of several 
feet. No doubt the colors, among the coral polyps and other 
life of the ancient seas, were as brilliant as now exist- 

Nature’s economist here puts the question—Why all this 
beauty when there were no eyes to enjoy it? But beauty ex- 
ists because, ‘in the beginning,” ‘the Spirit of God moved 
upon the face of the waters ;” and man finds delight therein in- 
asmuch as he bears the image of his Maker. 

A single recent species has been obtained by Mr. L. F. de 
Pourtales, in dredging at a depth of 324 fathoms, near the 
Florida reef, which may be a Cyathophylloid, although it has 
been supposed that the species of the tribe have been extinct 
since the middle of the Mesozoic era. It was half an inch 
high and broad, and the polyp-cell had eight septa—a mul- 
tuple of four, as in the true Cyathophylloids. The discoverer 
has named it //aplophyllia paradoxa. But he observes that 
it may after all be only an abnormal Actinoid. 


Il. ALCYONOID POLYPS. 


The name Alcyonewm, given to some of the species of this 
croup, is derived from Alcyone, the fabled daughter of Nep- 
tune. It is sometimes written with an initial H, in conform- 
ity with the aspirate of the Greek word; but Latin authors 
usually omitted the H, and this has been good enough author- 
ity for Linneus and the majority of later writers. 

The Alcyonoids include some of the gayest and most deli- 
cate of coral shrubs. Almost all are flexible, and wave with the 


motion of the waters. They contribute but little to the mate-. 


rial of coral reefs, but add largely to the beauties of the coral 
landscape. Not only are the polyps of handsome tints, but 
the whole shrub is usually of a brilliant orange, yellow, scarlet, 


ALCYONOID POLYPS. 81 


crimson or purple shade. Dun colors also occur, as ash- 
gray, and dark brown, and almost black. Some kinds, the 
Sponggodiz, are too flexible to stand erect, and they hang 
from the coral ledges, or in the coral caves, in gorgeous clus- 
ters of scarlet, yellow, and crimson colors. 

The species of this order spread from the tropics through 
the colder seas of the globe, and occur at various depths, down 
to thousands of feet. 

The two following are the most striking external peculiari- 
ties of the polyps: the number of tentacles is always eight; 
and these tentacles are always fringed with papille, though the 
papillz are sometimes mere warts. Some of the various forms 
of the polyps are shown in the figures on the following pages. 

But besides these characteristics, there is also the follow- 
ing: the existence of only eight internal septa, and these septa 
not in pairs; consequently, the interior is divided into only 
eight compartments (octants), and with each a tentacle is con- 
nected. Hence in the Alcyonoids, as Prof. Verrill has ob- 
served, the areas externally, and the compartments within, 
are all ambulacral, or tentacular, which makes a wide dis- 
tinction between them and the Actinoids (p. 28) in which only 
the alternate are tentacular. 

The solid secretions of these polyps are of two kinds: Ki- 
ther (1), internal and calcareous; or (2), epidermic, from the 
base of the polyp. The latter make an axis to the stem or 
branch, which is either horney (like that in Antipathus, p. 62) or 
calcareous. A few species have no solid secretions. 

Allthe species are incapable of locomotion on the base ; 
yet there are some that sometimes occur floating in the open 
ocean. 

The three following divisions of the Alcyonoids are those 


now generally recognized : 
6 


82 CORALS AND CORAL ISLANDS. 


1. The Alcyonium tribe or Axucyonacea.—One of the 
forms under this tribe is represented in the annexed figure. 
It is from the Feejees (like most of the zodphytes figured 
by the author), and in the living state the polyps had the mid- 
dle portion of the tentacles pale brown, with the fringe deep 
brown. In another more beautiful species of the genus, from 
the same region, the Xenia florida D. (made Xenia Dane by 
Verrill, as it proved to be distinct from Lamarck’s species to 





XENIA ELONGATA, D. 


which the author referred it), the polyps are as large, but short- 
er, and the color is a.shade of lilac. These species differ from 
the larger part of the Alcyonia in having the polyps not re- 
tractile; the tentacles fold together, if the zodphyte is disturb- 
ed, but cannot hide themselves. 

The following figure represents another related species 





ae 
oat 


7 1 eo | 


sea * 





1, 1a, Merulina pegalis; 2. 2a, Telesto trichostemma. 


2 TR Se eee ee 





MCZ LIBRARY 
HARVARD UNIVERSETY 
CAMBRIDGE. MA USA 


the 





ALCYONOID POLYPS. 83 


obtained by Dr. W. Stimpson, near Hong Kong, and called by 
its discoverer Anthelia lineata ; the polyps are but partly ex- 
panded. 

Other Alcyonoids are much branched, with the branches 
thick and finger-like, and soft or flexible, and the polyps small 
and wholly retractile into the mass. The branches, bare of 
polyps, are usually of some dull pale color, and on account of 
this fact some of these Alcyonia go by the common name of 
dead-men’s fingers. 





ANTHELIA LINEATA, ST. 


The above kinds secrete granules or spicules of carbonate 
of lime in the tissues, and are harsher or softer in texture ac- 
cording to the proportion of these granules. 

Some species form branching tubes, rising from an in- 
crusting base, which are rather firm owing to the calcareous 
spicules present. Such species are referred to the genus Te- 
lesto—one of which, from Hong Kong, from the collection 
made by Dr. Stimpson, is here figured (from Verrill). The 
second figure shows the form of the expanded polyps. 

Another species of Telesto, 7. trichostemma, is represented 
colored, as in life, in figures 1, 2a, on Plate VI. It encrusts 
the dead axis of a branching Antipathes. The polyp is re- 
markable for its size and beauty. 

In one family of this tribe the polyps form red calcareous 
tubes; sometimes a slender, creeping tube, with polyps at 
intervals, as in a species referred by the author to the venus 


54 CORALS AND CORAL ISLANDS. 


Aulopora; but generally vertical tubes, grouped into large red 
masses, called, popularly, Organ-pipe coral. A portion of one 
of the latter—Tubipora syrnga D.—is represented in the 





TELESTO RAMICULOSA, V. 


first of the following figures, with its expanded polyps; and a 
polyp from the group much enlarged in the second figure. 
The papilla of the fringe are arranged closely together in a 





9 


1, 2. TUBIPORA SYRINGA, D.; 3. T. FIMBRIATA, D. 


, 


plane, so that it is not at first apparent that there is a fringe. 
The third figure represents, enlarged, the polyp of another Fee- 
jee species, the Zubspora fimbriata D. Such coral masses 


.re sometimes a foot or more in diameter, and the living zo0- 


ALCYONOID POLYPS. 85 


phyte, with its lilac or purple polyps fully exparided, looks 
much like a large cluster of flowers from a lilac bush. The 
tubes are united by cross plates at intervals. 

2. Gorgonia tribe, or Gorconacea.—The following figure 
represents a species of this tribe from the Kingsmill or Gilbert 





GORGONIA FLEXUOSA, D. 


Islands. It is one of the net-like or reticulated species, the 
reticulation being a result of the coalescence of the branchlets. 
The general color of the species was crimson; but when alive 
and expanded it was covered throughout with yellowish polyps 
of the form in figure a, though much smaller, the natural size 
not exceeding a twelfth of an inch. The common sea-fan of 
the West Indies, Gorgonia flabellum, is much more finely 
reticulated, the meshes of the net-work being ordinarily not 
over a fourth of an inch in breadth; while the fan often grows 
to a height and breadth of a yard. 

Other species of the Gorgonia family are like clusters 


86 CORALS AND CORAL ISLANDS. 


of slender twigs, and others like many-branched shrubs or 
miniature trees. 

The exterior of the stem or branch in a Gorgonia is a 
layer of united polyps, with minute calcareous spicules dis- 
tributed through the tissues and giving the layer some firm- 
ness. Itislikea bark to the axis of the stem or branch, and may 
be peeled off without difficulty, and hence is often called the 
cortex. The outer surface of the dried cortex is often smooth, 
or nearly so; but sometimes covered with small prominences. 
Over it there may be seen numerous oblong points (one to 
each of the prominences if there are any; each of these is 
the spot where a polyp opened out its tentacles when the zo6- 
phyte was alive. 

Kolliker and others have shown that genera, and some- 
times species, of the Gorgonacea, may be distinguished by the 





SPICULES OF GORGONIZ, MUCH ENLARGED. 


forms of the calcareous spicules. Some of these knobby spi- 
cules are represented in the annexed cut, from figures published 
by Prof. Verrill. The most common forms are those of figures 
1, 4, 5; they occur, with small differences, in the genera Gor- 
gonia, Eugorgia, Leptogorgia, etc. Figure 1 is from the Lep- 
togorgia eximia V. Figure 2,in which one side is smooth 
(from the Gorgonia quercifolia V), is characteristic of the 
genus Gorgonia, but occurs in the species along with forms 


much like fig. 1. The forms represented in figures 3, 4, 5, 


ALCYONOID POLYPS. 87 


are all from Hugorgia aurantiaca V., the peculiar kind shown 
in fig. 3 occurring with the other more common form, in species 
of this genus. In species of Plexaurella many of the spi- 
cules are beautiful crosses of various fancy shapes. In Eu- 
nicellz the cortex is covered with an outside layer, in which 
the spicules are club-shaped, though ornately so, and have the 
smaller end pointed inward. These spicules afford valuable dis- 
tinguishing characters also in all Alcyonoids. 

The spicules are often brilliantly colored, and sometimes 
variously so in the same individual. Yellow, crimson, scar- 
let and purple are common colors, and they occur both of 
dark and pale shades. Viewed under a compound micro- 
scope by transmitted light, a group of these spicules from 
some species, part bright yellow and part crimson, or of 
some other tints, produces an exceedingly beautiful effect. 
It gives still greater interest to this subject that all Gor- 
goniz owe the various colors they present to the colors of 
their spicules. 

Spicules are usually wholly internal, or they only come to 
the surface so as to make the exterior slightly harsh. But in 
other cases, as in the genus Muricea, they project and give 
a somewhat bristly look to the coral. 

The calcareous spicules are internal secretions, like those 
of ordinary coral, and the constitution is the same,—mere 
carbonate of lime. But the secretion of the axis of the 
branches is epidermic, from the inner surface of the cortex, 
asin the Antipathus before described (p. 62). In the ordinary 
Aleyonoids that make no horny axis, the stolons, or budding 
stem or mass, creeps or spreads over the supporting body. 
But in these Gorgoniz, the budding cluster, which would make 
a stolon if there were no horny secretions, has the form of a 
tube about a horny axis; and as this tube elongates and se- 


88 CORALS AND CORAL ISLANDS. 


cretes the axis within, it gives out buds externally; thus the 
branch rises. New branches commence at intervals over the 
sides of the rising stem or branch through the starting of new 


TALE y) 
1 Mian 
Ui 





ISIS HIPPURIS, LINN. 


budding centres, and so, finally, the Gorgonia zodphyte is 
completed. 

In a few species, the axis is partly or wholly calcareous. 
In the Isis family, it is made up of a series of nodes and 
internodes. The former, in the genus Isis, are white, calcare. 
ous, furrowed or fluted pieces; and the latter are smaller and 
horn-like in nature, as illustrated in the preceding figures. 
In the branching stem here figured, the main stem and the 
branch on the left are simply the axis, bare of the polyp-layer 


ALCYONOID POLYPS. 89 


or cortex; while the branch on the right, with the surface 
dotted, has the cortex complete, and the dots are the sites of 
the contracted polyps. ‘The circular figure below is a trans- 
verse section of the stem enlarged, showing the cavities occu- 
pied by the retracted polyps. 

In the genus Melitza, and some others related, the inter. 





CORALLIUM RUBRUM. 


nodes are porous and somewhat cork-like or suberose instead 
of horny. ‘The species of this group are often bright-colored 
and much branched, and resemble, in aspect, ordinary Gor- 
gonie ; but they are very brittle, breaking easily at the 


internodes. 


90 VORALS AND CORAL ISLANDS. 


In the Corallide, the axis is wholly calcareous, and firm and 
solid throughout, with usually a red color, varying from crim- 
son to rose-red. Here belongs the Coralliwm rubrum, or pre- 
cious coral. The polyp-crust or cortex, which covers the red 
axis or coral, is thin, and contains comparatively few calcare- 
ous spicules, and consequently it readily disappears when the 
dried specimens are handled. In an uninjured state, the polyp 
centres may be distinguished over it by a faint six-rayed star. 
A branch from a specimen obtained by the author at Naples, 
is represented, of natural size, in the cut on page 89. The pol- 
yps, as the enlarged view, by Lacaze Duthiers, shows, are sim- 
ilar to those of other Alcyonoids—the tentacles being eight in 
number and fringed. The figure represents the extremity of 
a branch, magnified about four times lineally, with one polyp 
fully expanded, two partly, and the rest unexpanded. In the 
living Corallium, they open out thickly over the branches, and 
make it an exceedingly beautiful object. The coral grows in 
branching forms, spreading its branches nearly in a plane; and 
sometimes the little shrub is over a foot in height. The au- 
thor just mentioned states that, among the polyps, those of the 
same branch are often all of one sex alone, and that, besides 
males and females, there are a few that combine both sexes. 
The red caleareons axis consists really of united spicules. 

The precious coral is gathered from the rocky bottom of 
the borders of the Mediterranean, or its islands, and most 
abundantly at depths of 25 to 50 feet, though occurring 
also even down to 1,000 feet. There are important fisheries 
on the coast of southern Italy ; of the island of Ponza, off the 
Gulf of Gaeta; of Sicily, especially at Trapani, its western ex- 
tremity ; of Corsica and Sardinia, in the straits of Bonifacio ; 
of Algeria, south of Sardinia, near Bona, Oran, and other 
places, which in 1853 afforded 80,000 pounds of coral; and on 


ALCYONOID POLYPS. 91 


the coast of Marseilles. The rose-colored is the most highly 
valued, because the rarest. 

Another species of Corallium was obtained by the author 
at the Sandwich Islands (Atlas of Zodphytes, plate 60) ; but, 
while probably from the seas of that region, its precise locality 
is not known. 

3. Pennatula tribe, or Pennarunacea. These are com- 





COPHOBELEMNON CLAVATUM, V., AND VERETILLUM STIMPSONI, V. 


pound Alcyonoids, that, instead of being attached to rocks or 
some firm support, have the base or lower extremity free from 
polyps and buried in the sand or mud of the sea-bottom, or 
else live a floating life in the ocean. Their forms are very va- 
rious. 

In the Veretillum family (Veretillide) they are stout 
and short club-shaped. One of the species from Hong Kong, 
is shown in the figure on the left, with its polyps fully ex- 


92 CORALS AND CORAL ISLANDS. 


panded, and the small figure represents one of the polyps en- 
larged. The third figure represents a polyp of another spe- 
cies, from Hong Kong, a true Veretillum, enlarged three di- 
ameters; the specimens, obtained by Dr. Stimpson, and de- 
scribed by Prof. Verrill, were six to eight inches in length, and, 
where thickest, were three inches or more in diameter. 

A common Mediterranean species is the Veretillum cynomo- 
roum ; and it has been recently found, of a length of ten in- 
ches, in the depths of the Atlantic off the coast of Spain. Mr. 
W. S. Kent observes, with regard to its polyps and their 
phosphorescent qualities, as follows : 

‘“‘ Nothing can exceed the beauty of the elegant opaline pol- 
yps of this zodphyte when fully expanded, and clustered 
like flowers on their orange-colored stalk ; a beauty, however, 
almost equalled by night, when, on the slightest irritation, the 
whole colony glows from one extremity to the other with un- 
dulating waves of pale green phosphoric light. A large buck- 
etful of these Alcyonaria was experimentally stirred up one 
dark evening 


oO? 
spectacle too brilliant for words to describe. The supporting 


and the brilliant luminosity evolved produced a 


stem appeared always to be the chief seat of these phosphor- 
escent properties, and from thence the scintillations travelled 
onward to the bodies of the polyps themselves. Some of the 
specimens of this magnificent zodphyte measured as much as 
ten inches from the proximal to the distal extremity of the 
supporting stalk, while the individual polyps, when fully ex- 
serted, protruded upward of an inch and a half from this in- 
flated stalk, and measured as much as an inch in the diameter 
of their expanded tentacular discs.” 

In several genera of the Pennatula tribe there are two 
kinds of polyps over the surface, and this was the case with the 
Veretillum Stimpsoni, as observed by Prof. Verrill. Between 








ALCYONOID POLYPS. 93 


the large and well-developed polyps, there were multitudes 
of small wart-like prominences, each of which proved to be 
a polyp, but very small and imperfectly developed, having 
only two lamelle in the interior instead of the usual eight, 
and without distinct tentacles, or the ordinary nettling cords 
within. 

Among the other forms of Zoéphytes in the Pennatula tribe 
are those having a stout axis. with branches either side, 
arranged regularly in plume-like style (the Pennatulide); 
or a very slender stem and very short lateral polyp-bearing 
pinnules or processes along it (the Virgularide); or a thin 
reniform shape (Renillidz). Others differ from the preceding 
in having the polyps not retractile; and some of these have 
a slender stem and the polyps arranged along one side of it 
(the Pavonaridz) ; and still others a terminal cluster of polyps 
(the Umbellularide). 

The most of the species secrete a slender, horny axis, and 
have slender calcareous spicules among the tissues, somewhat 
like those of the Gorgonidz. By the thickened base of the 
stem these species anchor the corallum in the mud. Many 
species occur in the deep seas, some at depths of two thou- 
sand fathoms. Moreover, they are brilliantly phosphores- 
cent; and Moseley says that the depths may be in places 
lighted by patches of these species and “ possibly the animals 
with eyes congregate around these sources of light.” 

The Heliopore are peculiar among the Alcyonoids in havy- 
ing a solid compound corallum, of rather large size; and 
they are alone among corals in having a blue color within. 
The corallum consists of slender tubes with intervening cel- 
lular coenenchyma; and as the tubes are crossed by tabula, 
though distantly, Heliopora has been referred to the Tabu- 
late, and also to the Milleporids. It was shown to belong 


Q4 CORALS AND CORAL ISLANDS. 


here by Moseley of the Challenger Expedition. The eight 
tentacles are pinnately fringed as in other Alcyonoids, and 
are wholly retractile by mtroversion. /eliolites, a Paleozoic 
genus, is supposed to be related to Heliopora. The most 
common species of Heliopora is the //. cwrulea of the East 


Indies. 


IV. LIFE AND DEATH IN CONCURRENT PROGRESS IN CORAL 
ZOOPHYTES. 


The large, massive forms of stony corals would not exist, 
and the tree-shaped and other kinds would be of diminutive size, 
were it not for the fact that, in the living zodphyte, death and 
life are going on together, pari passu. This condition of 
growth is favored by the coral secretions; for these give a 
chance for the polyp to mount upward on the coral, as it 
lengthens it by secretions at the top. But, to be successful in 
this ascending process, either the polyp must have the power 
of indefinite elongation, or it must desert the lower part of 
the corallum as growth goes forward; and this last is what 
happens. In some instances, a polyp, but a fourth of an inch 
long, or even shorter, is finally found at the top of a stem 
many inches in height. ‘The following figure represents a case 
of this kind; for all is dead coral, excepting less than an 
inch at the extremity of each branch. The tissues that once 
filled the cells of the rest of the corallum have dried away, 
as increase went on above. Another example is shown 
on page 54, in which the living part had a length of one 
eighth of an inch. The Goniopora, on page 52, is still an- 
other example of the process; but here the living part com- 
bines a great number of polyps: these are growing and _ bud- 
ding with all the exuberance of life, while below, the old _pol- 


LIFE AND DEATH IN CONCURRENT PROGRESS. 95 


yps gradually disappear, and even their cells become superfi- 
cial and fade out. Trees of Madrepores may also have their 
limits—all below a certain distance from the summit being 
dead; and this distance will differ for different species. Bur 
this is not a limit to the existence of the zodthome, even 





CAULASTR#A FURCATA, D. 


though a slender tree or shrub, or of its flourishing state; for the 
dead coral below is firm rock itself, often stronger than ordinary 
limestone or marble, and serves as an ever-rising basement 
for the still expanding and rising zodphyte. 

But this death is not in progress alone at the base of the 
column orbranch. Generally the whole interior of a corallum 
is dead, a result of the same process with that just explained. 
Thus, a Madrepora, although the branch may be an inch in 
diameter, is alive only to the depth of a line or two, the grow- 
ing polyps of the surface having progressively died at the low- 
er or inner extremity as they increased outward. 

The large domes of Astrzeas, which have been stated to 
attain sometimes a diameter of ten or fifteen feet, and are 


96 CORALS AND CORAL ISLANDS. 


alive over the whole surface, owing to a symmetrical and un- 
limited mode of budding, are nothing but lifeless coral 
throughout the interior. Could the living portion be sepa- 
rated, it would form a hemispherical shell of polyps, in most 
species about half an inch thick. In some Porites of the same 
size, the whole mass is lifeless, excepting the exterior for a 
sixth of an inch in depth. 

With such a mode of increase, there is no necessary limit 
to the growth of zodphytes. The rising column may increase 
upward indefinitely, until it reaches the surface of the sea, and 
then death will ensue simply from exposure, and not from any 
failure in its powers of life. The huge domes may enlarge till 
the exposure just mentioned causes the death of the summit, 
and leaves only the sides to grow, and these may still widen, 
it may be indefinitely. Moreover, it is evident that if the 
land supporting the coral domes and trees were gradually 
sinking, the upward increase might go on without limit. 

In the following of death after life ‘“ aquo pede,” there is 
obedience to the universal law. And yet the polyps, through 
this ever yielding a little by piecemeal, seem to get the better 
of the law, and in some instances secure for themselves almost 
perpetual youth, or at least a very great age. Of the polyps 
over an Astraea hemisphere, none ever die as long as the dome 
is in a condition of growth; and the first budding individual, 
or at least its mouth and stomach, is among the tens of 
thousands that constitute the living exterior of the dome of 
fifteen feet diameter. In the Madrepore, the terminal parent- 
polyp of a branch grows on without being reached by the 
death-warrant that takes off at last the commoners about the 
base of the tree; it keeps growing and budding, and the tree 
thus continues its increase. 

The death of the polyps about the base of a coral tree 


PROTECTION AGAINST INJURY. Sel 


_ would expose it, seemingly, to immediate wear from the waters 
around it, especially as the texture is usually porous. 
But nature is not without an expedient to prevent to some 
extent this catastrophe. 

In the first place, there is often a perdtheca over the 
dead corallum—that is, an outer impervious layer of carbonate 
of lime, secreted by the lower edge of the series of dying pol- 
yps, a fact in the Goniopora columna figured on page 52. 
Then, further, the dead surface becomes the resting-place of 
numberless small encrusting species of corals, besides Nulli- 
pores, Serpulas, and some Mollusks. In many instances, the 
lichen-like Nullipore grows at the same rate with the rate of 
death in the zodphyte, and keeps itself up to the very limit of 
the living part. The dead trunk of the forest becomes covered 
with lichens and fungi, or in tropical climes, with other foliage 
and flowers; so among the coral productions of the sea, there 
are forms of life which replace the dying polyp. The process 
of wear is frequently thus prevented. 

The older polyps, before death, often increase their coral se- 
cretions also within, filling the pores as the tissues occupying 
them dwindle, and thus render the corallum nearly solid; and 
this is another means by which the trees of coral growth, 
though of slender form, are increased in strength and endur- 
ance. 

The facility with which polyps repair a wound, aids in 
carrying forward the results above described. The breaking 
of a branch is no serious injury to a zoodphyte. ‘There is often 
some degree of sensibility apparent throughout a clump even 
when of considerable size, and the shock, therefore, may occa- 
sion the polyps to close. But, in an hour, or perhaps much 
less time, their tentacles will again have expanded; and such 
as were torn by the fracture will be in the process of com- 


- 
i 


98 CORALS AND CORAL ISLANDS. 


plete restoration to their former size and powers. The frag- 
ment broken off, dropping in a favorable place, would become 
the germ of another coral plant, its base cementing by means 
of new coral secretions to the rock on which it might rest; or, 
if still in contact with any part of the parent tree, it would be 
reunited and continue to grow as before. The coral zodphyte 
may be levelled by transported masses swept over it by the 
waves; yet, like the trodden sod, it sprouts again, and contin- 
ues to grow and flourish as before. The sod, however, 
has roots which are still unhurt; while the zodphyte, which 
may be dead at base, has a root—a source or centre of life—in 
every polyp that blossoms over its surface. Hach animal 
might live and grow if separated from the rest, and would ul- 
timately produce a mature zodphyte. 


V. COMPOSITION OF CORAL. 


Ordinary corals have a hardness a little above that of com- 
mon limestone or marble. The ringing sound given, when cor- 
al is struck with a hammer, indicates this superior hardness. 
It is possible that it may be owing to the carbonate of lime be- 
ing in the state of aragonite, whose hardness exceeds a little 
that of ordinary carbonate of lime or calcite. It is a common 
error of old date to suppose that coral when first removed from 
the water is soft, and afterward hardens on exposure. For, in 
fact, there is scarcely an appreciable difference; the live coral 
may have a slimy feel in the fingers ; but if washed clean of the 
animal matter, it is found to be quite firm. ‘The waters with 
which it is penetrated may contain a trace of lime in solution, 
which evaporates on drying, and adds slightly to the strength 
of the coral ; but the change is hardly appreciable. A branched 
Madrepore rings on being struck when first collected ; and a 
blow in any part puts in hazard every branch throughout it, 


COMPOSITION OF CORALS. 99 


on account of its elasticity and brittleness. The specific gravi- 
ty of coral varies from 2°5 to 2°8: 2523 was the average from 
fifteen specimens examined by Prof. Silliman. 

Chemically, the common reef-corals, of which the branch- 
ing Madrepora and the massive Astreas are good exam- 
ples, consist almost wholly of carbonate of lime, the same in- 
gredient which constitutes ordinary limestone. In 100 parts, 
95 to 98 parts are of this constituent ; of the remainder, there 
are 14 to 4 parts of organic matter, and some earthy ingredi- 
ents amounting usually to less than 1 per cent. These earthy 
ingredients are phosphate of lime, with sometimes a trace of 
silica. A trace of fluorine also has been observed. 

S. P. Sharples found the following constitution for the spe- 
cies below named (Am. Jour. Scz., IIIL., 1. 168). 


CARBONATE PHOSPHATE WATER AND OR- 


OF LIME. OF LIME. GANIC MATTERS. 
Ocalina arbuscula, N. Car.. . . 95.37 . . 0.84 . . . 38.79 
Manicina areolata, Florida. . . 96.54 . . 0.50 . . . 2.96 
Agaricia agaricites ieee eo Tao. |). 0.8) OLDS! a 20.5 | cee Mal 4 
Siderastrea radians Meme O30) cos, MOLLBe sree stn (eee 
Meaitopora cervicornis . /. . .- 98.07 . .. 0.82 . . = 1.93 
Madrepora palmata Mee aM OTTO et it OL 10h ty aif swans Ol 


Forchhammer found 2:1 per cent. of magnesia in Coral- 
lium rubrum, and 6°36 in Isis hippuris. 

The sea-water, and the ordinary food of the polyps, are evi- 
dently the sources from which the ingredients of coral are ob- 
tained. The same powers of elaboration which exist in other 
animals belong to polyps; for this function, as has been re- 
marked, is the lowest attribute of vitality. Neither is it at all 
necessary to inquire whether the lime in sea-water exists as 
carbonate, or sulphate or whether chloride of calcium takes the 
place of these. The powers of life may make from the ele- 


100 CORALS AND CORAL ISLANDS. 


ments present whatever results the functions of the animal re- 
quire. 

The proportion of lime salts which occurs in the water of 
the ocean is about | to % of all the ingredients in solution. The 
lime is mainly in the state of sulphate. Bischof states that the 
proportion of salts of all kinds in sea-water averages 3°527 per 
cent.; and in 100 parts of this, 75°79 are chloride of sodium, 
916 chloride of magnesium, 3°66 chloride of potassium, 1°18 
bromide of sodium, 4°62 sulphate of lime or gypsum, and 5°597 
sulphate of magnesia,=100. This corresponds to about 164 
parts of sulphate of lime to 10,000 of water. 

Fluorine has also been detected in sea-water; so that all the 
ingredients of coral are actually contained in the waters of the 
ocean. 

It has been common to attribute the origin of the lime of 
corals to the existence of carbonic-acid springs in the vicinity 
of coral islands. But it is an objection to such a hypothesis, 
that, in the first place, the facts do not require it; and, in the 
second, there is no foundation for it. The islands have been 
supposed to rest on volcanic summits, thus making one hy- 
pothesis the basis of another. Carbonic-acid springs are by no 
means a universal attendant on volcanic action. The Pacific 
affords no one fact in support of such an opinion. ‘There are 
none on Hawaii, where are the most active fires in Polynesia; 
and the many explorations of the Society and Navigator Isl- 
ands have brought none to light. Some of the largest reefs 
of the Pacific, those of Australia and New Caledonia, oc- 
cur where there is no evidence of former volcanic action. 

The currents of the Pacific are constantly bearing new sup- 
plies of water over the growing coral beds, and the whole ocean 
is thus engaged in contributing to their nutriment. Fish, mol- 
lusks, and zoéphytes are thus provided with earthy ingredi- 


HYDROIDS. 101 


ents for their calcareous secretions, if their food fails of giving 
the necessary amount; and, by means of the powers of animal 
life, bones, shells, and corals alike are formed. 

The origin of the lime in solution throughout the ocean is 
an inquiry foreign to our present subject. It is sufficient here 
to show that this lime, whatever its source, is adequate to ex- 
plain all the results under consideration. 


I. HYDROIDS. 


The annexed sketch represents a Hydra as it often occurs 
attached to the under surface of a floating leaf—that of a spe- 
cies of Lemna. The animal is seldom over half an inch 










































































long. It has the form of a polyp, with long slender tentacles ; 
and, besides these tentacles with their lasso-cells, it has no spe- 
cial organs except a mouth and a tubular stomach. Like the 
fabled Hydra, if its head be cut off another will grow out; and 
any fragment will, in the course of a short time, become a per- 
fect Hydra, supplying head, or tail, or whatever is wanting: 
and hence the name given to the genus by Linnzus. 


102 CORALS AND CORAL ISLANDS. 


The Hydroids were long considered polyps. But they 
have been found to give origin, with few exceptions, to Weduse, 
or jelly-fishes, and it is now proved that they are only an 
intermediate stage in the development of Medusz, between 
the embryo state and that of the adult or Medusa state. The 
Millepores afford, therefore, examples of coral-making by spe- 
cies of the class of Acalephs. Many of these Meduse and 
their Hydroids will be found illustrated in the admirable work 
of Alexander and Mrs. L. Agassiz entitled “ Sea-Side Studies,” 
—an excellent companion for all who take pleasure in sea- 
shore rambles. 

The Hydra is the type of a large group of species. It buds, 
but the buds drop off soon, and hence its compound groups 
are always small, and usually it is single. But other kinds 
multiply by buds that are persistent, and almost indefinitely 
so; and they thus make membranous coralla of considerable 
size and often of much beauty. 

The species here figured, Hydrallmania falcata (formerly 
called Plumularia falcata), is one of them. Along the 
branches there are minute cells, each of which was the seat of 
one of the little Hydra-like animals (in this not a fourth of a 
line long) having usually short tentacles spread out star-like. 
Other kinds are simple branching threads, and sometimes the 
cells are goblet-shaped and terminal. The Tubulariz grow 
in tufts of thread-like tubes, and have a star-shaped flower 
at top often half an inch in diameter, with a proboscis-like 
mouth at the centre. In Coryne, a closely-related genus, the 
tentacles are shorter, and somewhat scattered about the club- 
shaped or probosciform head of the stem, so that the animal 
at top is far from star-shaped or graceful in form. 

To the animal of the Coryne, that of the very common, and 
often large, corals, called Millepores, is closely related, as first 





HYDROIDS. 103 
detected by Agassiz on one of his cruises to the reefs of Flori- 
da. The coral-making Hy 


rdroids have been named /7ydroco- 





HYDRALLMANIA FALCATA, 


ralline. The group includes also, as Moseley first showed, the 


Stylasteridz mentioned on page 70 and other related species. 


The corals of the Milleporz are solid and stony, as much 


so as any in coral seas. They have generally a smooth sur- 


104 CORALS AND CORAL ISLANDS. 


face, and are always without any prominent calicles, there 
being only very minute rounded punctures over the surface, 
from which the animals show themselves. The cells in the 
corallum are divided parallel to the surface, but irregularly, 
by very thin plates or tables, approaching in this character 
the Pocilliporee and Favosites. 

Each coral is a group or colony of Hydroids in the Hydri- 
form state. Agassiz observes that the animals of Millepora 
are very slow in expanding themselves. When expanded, they 


have no resemblance to true polyps; there is simply a fleshy 





ANIMALS OF MILLEPORA ALCICORNIS, MUCH ENLARGED. 


tube with a mouth at top and a few small rounded promi- 
nences in place of tentacles, four of them sometimes largest. 
The preceding figure, from Agassiz, shows, much enlarged, a 
portion of a branch of the AMillepora alcicornis with the ani- 
mals expanded; and the small figure a, near the top of the 
cut, gives the natural size of the same. But it has been fur- 
ther observed by Moseley that in the Millepores the animals 
have two forms: one is the tentacle-like kind here figured, 
and the other shorter and mouth-bearing; and the former 
are sometimes arranged around the latter in more or less 


perfectly circular groups, called “ cyclosystems.” 


BRYOZOANS. 105 


In the Stylasteride, the cyclosystems occupy usually stel- 
late cells which stand out prominently along the branches, and 
look much like the calicles of an Oculina (Figs. 2, 3, p. 69). 

8. P. Sharples found the coral of M. alcicornis to consist 
of 97:46 per cent of carbonate of lime, 0:27 of phosphate of 
lime, and 2°54 of water and organic matters. The Millepores 


contribute largely to the material of coral reefs. 





MILLEPORA ALCICORNIS, LINN. 


The ancient corals of the Cheetetes family may be Hydro- 
coralline, as suggested by Agassiz, but more probably were 
Bryozoan. 

Ill. BRYOZOANS. 


The Bryozoans are very small animals, and look much like 
Hydroids. Although belonging to the sub-kingdom of Mol- 


lusks, they are externally polyp-like, having a circle or ellipse of 


106 CORALS AND CORAL ISLANDS. 


slender tentacles around the mouth. But, in internal struc- 
ture, and all of the animal below the head, they are Mollusks. 
They form delicate corals, membranous or calcareous, made up 
of minute, cabin-like cells, which are either very thin crusts on 
sea-weeds, rocks, or other supports, or slender moss-like tufts, 
or graceful groups of thin, curving plates, or net-like fronds ; 
and sometimes thread-like lines, or open reticulations. 

Occasionally they make large, massive corals, from the 
growing of plate over plate. 

The first of the following figures, represents one of the 
delicately branching species, of natural size; and the second, 
a portion of the same, much enlarged. The latter figure 
shows that the branches are made up of minute cells. From 
each cell, when alive, the bryozoum extends a circlet of ten- 


tacles, less than a line in diameter. 





1, 2, HORNERA LICHENOIDES; 3, DISCOPORA SKENEI, SMITT. 


The encrusting kinds are common in all seas. The crust 
of cells they make is often thinner than paper. A portion of 
such a crust is represented, enlarged, in figure 3. When ex- 
panded, the surface is covered over with the delicate flower-like 
bryozoa. A low magnifying power is necessary to observe them 


NULLIPORES. 107 


distinctly. The animals, unlike true polyps and the Hydroids, 
have two extremities to the alimentary canal, and in this, and 
other points, they are Molluscan in type. 

The cells of a group never have connection with a common 
tube, as in the Hydroids; on the contrary, each little Bryozoum, 
in the compound group or zoédthome, is wholly independent of 
the rest in its alimentary canal. 

Bryozoans occur in all seas and at all depths; and in early 
Paleozoic time they contributed largely to the making of lime- 
stone strata, 


IV. NULLIPORES. 


The more important species of the Vegetable Kingdom that 
afford stony material for coral reefs are called Nullipores. 
They are true Algze or sea-weeds, although so completely stony 
and solid that nothing in their aspect is plant-like. They form 
thick, or thin, stony incrustations over surfaces of dead corals, 
or coral rock, occasionally knobby or branching, and often 
spreading lichen-like. 

They have the aspect of ordinary coral, especially the Mil- 

lepores, but may be distinguished from these species by their 
having no cells, not even any of the pin-punctures of those 
species. 
Besides the more stony kinds, there are delicate species, of- 
ten jointed, called Corallines, which secrete only a little lime in 
their tissues, and have a more plant-like look. Even these 
grow so abundantly on some coasts, that, when broken up and 
accumulated along the shore by the sea, they may make thick 
calcareous deposits. Agassiz has described such beds as hav- 
ing considerable extent in the Florida seas. 


108 CORALS AND CORAL ISLANDS. 


V. THE REEF-FORMING CORALS AND THE CAUSES INFLU- 
ENCING THEIR GROWTH AND DISTRIBUTION. 


I. DISTRIBUTION IN LATITUDE. 


Reef-forming species are the warm-water corals of the 
globe. A general survey of the facts connected with the tem- 
perature of the ocean in coral-reef seas appears to sustain the 
conclusion that they are confined to waters which, through even 
the coldest winter month, have a mean temperature not below 
68° F. Under the equator, the surface waters in the hotter 
part of the ocean have the temperature of 85° F’. in the Pacific, 
and 83° F. inthe Atlantic. The range from 68° F. to 85° F-. 
is, therefore, not too great for reef-making species. 

An isothermal line, crossing the ocean where this winter- 
temperature of the sea is experienced, one north of the equator, 
and another south, bending in its course toward or from the 
equator wherever the marine currents change its position, 
will include all the growing reefs of the world; and the area 
of waters may be properly called the coral-reef seas. 

This isothermal boundary line, the isocryme (or cold-water 
line) of 68° F., extends, through mid-ocean, near the parallel of 
28°; but in the vicinity of the continents it varies greatly from 
this, as explained beyond in the course of remarks on the geo- 
graphical distribution of reefs. It is to be observed that the 
temperature of 68° F. is a temporary extreme—not that under 
which the polyps will flourish. Except for a short period, the 
waters near the limits of the coral seas are much warmer; the 
mean for the year is about 734° F. in the North Pacific, and 
70° F. in the South; from which it may be inferred that the 
summer mean would be as high at least as 78° and 74° F. 

Over the sea thus limited coral reefs grow luxuriantly, yet 


GEOGRAPHICAL DISTRIBUTION OF UORALS. 109 


in greatest profusion and widest variety through its hotter por- 
tions. Drawing the isocryme of 74° F’. (that is, the isotherm 
for 74° F. as the mean for the coldest month) around the 
globe, the coral-reef seas are divided, both north and south of 
the equator, into two regions, a torrid, and a subtorrid, as thev 
are named by the author (see Chart beyond, from the Author’s 
Report on Crustacea); and these correspond, as seen below, 
to a marked difference in the corals which they grow. 

Further, the torrid region should be divided, as the distri- 
bution of corals show, into a warmer and a cooler torrid, the 
isocryme separating the two being probably that of 78°. 

But, before considering the facts connected with the geo- 
graphical distribution of existing coral-reef species, it is impor- 
tant to have a correct apprehension of what are these reef spe- 
cies as distinct from those of colder and deeper seas, 

The coral-reef species of corals are the following.— 

1. In the Astrea tribe (Astrzeacea), all the many known 
species. - 

2. In the Fungia tribe (Fungacea), almost all known spe- 
cies, the only exceptions at present known being two free spe- 
cies found much below coral-reef depths, in the Florida seas, 
by 0. I’. de Pourtales, one of them, at a depth of 450 fathoms. 


3. In the Oculina tribe (Oculinacea), all of the Orbicellids; ‘ 


part of the Oculinids and Stylasterids ; some of the Caryophyl- 
lids, Astrangids and Stylophorids; all of the Pocilloporids. 

4, Inthe Madrepora tribe (Madreporacea), all of the Madre- 
porids and Poritids; many of the Dendrophyllia family or 
Eupsammids. 

5. Among Alcyonoids, numerous species of the Aleyonium 
and Gorgonia tribes, and some of the Pennatulacea. 

6. Among Hydroids, the Millepores and Stylasterids. 

7. Among Algze, many Nullipores and Corallines. 


110 CORALS AND CORAL ISLANDS. 


The corals of colder waters, either outside of the coral-reet 
seas, or at considerable depths within them, comprise, accord- 
ingly, the following :— 

1. A very few Fungids. 

2. Some of the Oculinids; many of the Astrangids and 
Caryophyllids; a few Stylophorids. 

3. Many of the Eupsammids. 

4. Some of the Gorgonia and Pennatula tribes, and a few 
of the Aleyonium tribe. 

5. Milleporids of the genus Pliobothrus; many Stylasterids. 

A large proportion of the cold water species are solitary 
polyps. 

Through the torrid region, in the central and western Pa- 
cific, that is, within 15° to 18° of the equator, where the tem- 
perature of the surface is never below 74°F. for any month of 
the year, all the prominent genera of reef-forming species are 
abundantly represented—those of the Astraacea, Fungacea, 
Oculinacea, Madreporacea, Aleyonoids, Millepores and Nulli- 
pores. The Feejee seas afford magnificent exainples of these 
torrid region productions. Astras and Meandrinas grow 
there in their fullest perfection; Madrepores add flowering 
shrubbery of many kinds, besides large vases and spreading 
folia; some of these folia over six feet in expanse. Musse 
and related species produce clumps of larger flowers ; Meru- 
line, Echinopore, Gemmipore and Montipore form groups 
of gracefully infolded or spreading leaves; Pavoniz, Pocilli- 
pore, Seriatopore and Porites branching tufts of a great vari- 
ety of forms; Tubipores and Xenie, beds or masses of the 
most delicately-tinted pinks; Sponggodiz, large pendant clus- 
ters of orange and crimson; and Fungie display their broad 
disks in the spaces among the other kinds. Many of the 
species may be gathered from the shallow pools about the reefs. 


GEOGRAPHICAL DISTRIBUTION OF CORALS. itt 


But with a native canoe, and a Feejee to paddle and dive, the 
scenes in the deeper waters may not only be enjoyed, but boat- 
loads of the beautiful corals be easily secured. 

The Hawaian Islands, in the north Pacific, between the 

latitudes 19° and 22°, are outside of the torrid zone of oceanic 
| temperature, in the sudtorrid, and the corals are consequently 
less luxuriant and much fewer in species. There are no Mad- 
repores, and but few of the Astrea and Fungia tribes; while 
there is a profusion of corals of the hardier genera, Porites and 
Pocillipora. 

The genera of corals occurring in the East Indies and Red 
Sea are mainly the same as in the Central Pacific; and the 
same also occur on the coast of Zanzibar. 

At the eastern of the Pacific coral islands, the Paumotus, 
which are within the limits of the torrid region, the variety of 
species and genera is large, but less so than to the westward. 
Special facts respecting this sea have not been obtained. The 
author’s observations were confined to the groups of islands 
farther west, the department of corals having been in the hands 
of another during the earlier part of the cruise of the Govern- 
ment Expedition with which he was connected. 

The Gulf of Panama and the neighboring seas, north to the 
extremity of the California peninsula and south to Guayaquil, 
lie within the torrid region; but in the cooler part of it. The 
species have throughout a Pacific character, and nothing of 
the West Indian; but they are few in number, and are much 
restricted in genera. There are none, yet known, of the As- 
treeacea, and no Madrepores. Prof. Verrill, through the study 
of collections made by F. H. Bradley and others, has observed 
that there are, near Panama, a few species of Porites and Den- 
drophylliz, a Stephanaria (near Pavonia), two species of Po- 
cilliporz, two of Pavonis, one of them very large and named 


12 CORALS AND CORAL ISLANDS. 


P. gigantea V., several Astrangids, and a few other small 
species, besides a large variety under the Gorgonia tribe. At 
La Paz, on the California peninsula at the entrance to the Gulf, 
occur a small but beautiful Fungia (7. elegans V.), three Pori- 
tes, a Dendrophyllia, a Pocillipora, some Astrangids, and 
many fine Gorgonie. The character of the species is that. of 
the cooler torrid region, rather than that of the warmer 
torrid. 

Owing to the cold oceanic currents of the eastern border 
of the Pacific—one of which, that up the South American 
coast, is so strong and chilling as to push the southern isocryme 
of 68°, the coral-sea boundary, nearly to the Galapagos, 
and north of the equator—the coral-reef sea, just east of Pan- 
ama, is narrowed to 20°, which is 36° less of width than it has 
in mid ocean; and this suggests that these currents, by their 
temperature, as well as by their usual westward direction, have 
proved an obstacle to the transfer of mid-ocean species to the 
Panama coast. 

In the West Indies the reefs lie within the limits of the 
isocryme of 74° F., or the torrid region; and yet the variety 
of species and genera is very small compared with the same in 
the central Pacific. The region contains some large Madre- 
pores, the M. palmata, a spreading foliaceous species that 
forms clumps two yards in diameter; JL. cervicornis, a stout, 
sparsely-branched tree-like species, which attains a height of 
fifteen feet; JL prolifera, a handsome shrub-like species, of rathi- 
er crowded branches; besides others ; and these are marks of the 
existence of the warmer torrid region; yet the sea has not as 
high a temperature as the hottest part of the Pacific. The species 
of the Astraa tribe are few in number, and among the largest 
kinds are the Meandrine (the Diploria being here included). 
None of the free Fungide are known excepting the two spe- 


GHOGRAPHICAL DISTRIBUTION OF CORALS. 113 


cies In deep water, and none of the Pavonie among the com- 
pound species; but the massive Siderine (Siderastraez) are 
common, and the foliaceous Agaricie and Mycedia. Of the 
Oculina tribe, species of Oculina, Cladocora and Astrangia are 
relatively more numerous than in the central Pacific; but 
there are none of the Pocilliporids, which are common both in 
the torrid and subtorrid regions of the Pacific. Millepores are 
very common. Gorgoniz, are of many species. 

Prof. Verrill observes that not a single West Indian coral 
occurs on the Panama coast, although, on the opposite coast, at 
Aspinwall, there are found nearly all the reef-building species 
of Florida, viz.: Porites astrwoides Lmk., P. clavaria Luwnk., 
Madrepora palmata V.., M. cervicornis L., M. prolifera L., 
Meandrina clivosa V., M. labyrinthica, M. sinuosa Les., with 
other species of Mzandrina, Manicina areolata Ehr., Sider- 
astrea (Siderina) radiata V., S. galaxea Bl., Agaricia agari- 
cites, Orbicella cavernosa V., O. annularis D. Moreover no 
West Indian species is known to be identical with any from 
the Pacific or Indian ocean. 

The reefs of the Brazilian coast south of Cape Roque lie in 
the subtorrid region of oceanic temperature, or between the is- 
ocrymes ot 74° and 68°. The reef corals extend as far south 
as Cape Frio, according to Prot. C. F. Hartt. The species, as 
determined by Prof. Verrill, from Prof. Hartt’s collections, re- 
semble the West Indian. All species of Madrepora, Mzan- 
drina, Diploria, Manicina, Oculina, genera eminently charac- 
teristic of the West Indies, appear to be wanting, while the 
most important reef-making genera are Huvia, Acanthastrea, 
Orbicella, Siderastrea, Porites, and Millepora, and also, of 
less importance, Mussa and some others. A few species, viz. : 
Siderastrea stellata V., Orbicella aperta V., Astrea gravida 


V., and Porites solida V., are very close to West Indian spe- 
8 


114 CORALS AND CORAL ISLANDS. 


cies; and Millepora alcicornis is an identical species, though 
different in variety. 

The Bermudas are in the North Atlantic subtorrid region, 
in the range of the Gulf Stream. The few reef-making spe- 
cies that occur there are all West Indian. The principal among 
them are: Lsophyllia dipsacea, f. rigida, Astrea ananas, Di- 
ploria cerebriformis, D. Stokesi, Meandrina labyrinthica, M. 
strigosa, Orbicella cavernosa, Oculina diffusa, Oculina varicosa, 
Oculina pallens, Oculina Valenciennes, O. speciosa, Siderastreea 
radians, Mycedium fragile, Porites clavaria, P. astreoides, 
Millepora alcicornis ; and the common West India Aleyonoids, 
Gorgoma flabellum, Plexaura crassa Lx., Pl. flecuosa Lx., Pl. 
homomalla Lx., Pterogorgia Americana EKhr., Pt. acerosa Ehr. 

The facts presented are sufficient to show that temperature 
has much to do with the distribution of reef-corals in latitude, 
while proving also that regional peculiarities exist that are not 
thus accounted for. 


UW. DISTRIBUTION IN DEPTH. 


(uoy and Gaymard were the first authors who ascertained 
that reef-forming corals were confined to small depths, contrary 
to the account of Foster and the early navigators. The mis- 
take of previous voyagers was a natural one, for coral reefs 
were proved to stand in an unfathomable ocean; yet it was 
from the first a mere opinion, as the fact of corals growing at 
such depths had never been ascertained. The few species which 
are met with in deep waters appear to be sparsely scattered, 
and nowhere form accumulations or beds. 

The above-mentioned authors, who explored the Pacific in 
the Uranie under D’Urville (and afterward also in the As- 
trolabe), concluded from their observations that five or six 
fathoms (30 or 86 feet) limited their downward distribution. 


RANGE IN DEPTH OF CORALS. 115 


Ehrenberg, by his observations on the reefs of the Red Sea, 
confirmed the observations of Quoy and Gaymard; he conclud- 
ed that living corals do not occur beyond six fathoms. Mr. 
Stutchbury, after a visit to some of the Paumotus and Tahiti, 
remarks, in Volume I. of the West of England Journal, that 
the living clumps do not rise from a greater depth than 16 or 
17 fathoms. 

Mr. Darwin, who traversed the Pacific with Captain Fitz- 
roy, R. N., gives 20 fathoms as not too great a range. 

In his soundings off the fringing reefs of Mauritius, in 
the Indian ocean, on the leeward side of the island, he ob- 
served especially two large species of Madrepores, and two 
of Astrea; and a Millepora down to fifteen fathoms, with 
also, in the deeper parts, Seriatopora; between fifteen and 
twenty fathoms a bottom mostly of sand, but partly covered 
with the Seriatopora, with a fragment of one of the Madre- 
pores at twenty fathoms. He states that Capt. Moresby, in 
his survey of the Maldives and Chagos group, found, at seven 
or eight fathoms, great masses of living coral; at ten fathoms, 
the same in groups with patches of white sand between ; and, 
at a little greater depth, a smooth steep slope without any 
living coral; and further, on the Padua Bank, the northern 
part of the Laccadive group, which had a depth of twenty-five 
to thirty-five fathoms, he saw only dead coral, while on other 
banks in the same group.ten or twelve fathoms under water, 
there was growing coral. 

In the Red Sea, however, according to Capt. Moresby and 
Lient. Wellstead, there are, to the north, large beds of living 
corals at a depth of twenty-five fathoms, and the anchors were 
often entangled by them; and he attributes this depth, so 
much greater than reported by Ehrenberg, to the peculiar pu- 
rity, or freedom from sediment, of the waters at that place. Kot- 


116 CORALS AND CORAL ISLANDS. 


zebue states that in some lagoons of the Marshall group he ob. 
served living corals at a depth of twenty-five fathoms, or one 
hundred and fifty feet. 

Prof. Agassiz observes that about the Florida reefs, the 
reef-building corals do not extend below 10 fathoms. Mr. L. 
F. de Pourtales states that he found species of Oculina and Clad- 
ocora off the Florida reefs living to a depth of 15 fathoms. 

It thus appears that all recent investigators since Quoy and 
Gaymard have agreed in assigning a comparatively small depth 
to growing corals. The observations on this point, made dur- 
ing the cruise of the Wilkes Exploring Expedition, tend to 
confirm this opinion. 

The conclusion is borne out by the fact that soundings in the 
course of the various and extensive surveys afford no evidence 
of growing coral beyond twenty fathoms. Where the depth 
was fifteen fathoms, coral sand and fragments were almost uni- 
formly reported. Among the Feejee Islands, the extent of 
coral-reef grounds surveyed was many hundreds of square 
miles, besides the harbors more carefully examined. The reefs 
of the Navigator Islands were also sounded out, with others 
at the Society Group, besides numerous coral islands; and 
through all these regions no evidence was obtained of corals 
living at a greater depth than fifteen or twenty fathoms. 
Within the reefs west of Viti Lebu and Vanua Lebu, the anchor 
of the Peacock was dropped sixty times in water from twelve 
to twenty four fathoms deep, and in no case struck among 
growing corals; it usually sunk into a muddy or sandy bottom. 
Patches of reef were encountered at times, but they were at a 
less depth than twelve fathoms. By means of a drag, occasion- 
ally dropped in the same channels, some fleshy Alcyonia 
and a few Hydroids were brought up, but no reef-forming 
species. 


RANGE IN DEPTH OF CORALS. 1s 


Outside of the reef of Upolu, corals were seen by the writer 
growing in twelve fathoms. Lieutenant Emmons brought up 
with a boat-anchor a large Dendrophyllia from a depth of 
fourteen and a half fathoms at the Feejees; and this species 
was afterward found near the surface. But Dendrophyllia, it 
may be remembered, is one of the deep-water genera. 

These facts, it may be said, are only negative, as the sound- 
ing-lead, especially in the manner it is thrown in surveys, would 
fail of giving decisive results. The character of a growing 
coral bed is so strongly marked in its uneven surface, its deep 
holes and many entangling stems, to the vexation of the sur- 
veyor, that in general the danger of mistake is small. But al- 
lowing uncertainty as great as supposed, there can be little 
doubt as to the general fact after so numerous observations 
over so extended regions of reefs. 

The depth of the water in harbors and about shores where 
there is no coral, confirms the view here presented. At Upo- 
lu, the depth of the harbors varies generally from twelve to 
twenty fathoms. On the south side of this island, off Falealili, 
one hundred yards from the rocky shores, Lieutenant Perry 
found bare rocks in eighteen and nineteen fathoms, with no ev- 
idence of coral. There is no cause here which will explain the 
absence of coral, except the depth of water; for corals and 
coral reefs abound on most other parts of Upolu. Below Fa- 
lelatai, of the same island, an equal depth was found, with no 
coral. Off the east cape of Falifa harbor, on the north side of 
Upolu, Lieutenant Emmons found no coral, although the depth 
was but eighteen fathoms. About the outer capes of Funga- 
sa harbor, Tutuila, there was no coral, with a depth of fifteen 
to twenty fathoms; and a line of soundings across from capc 
to cape, afforded a bottom of sand and shells, in fifteen to 
twenty-one and a half fathoms. About the capes of Oafonu 


118 CORALS AND CORAL ISLANDS. 


harbor, on the same island, there was no coral, with a depth 
of fifteen fathoms. 

Similar results were obtained about all the islands surveyed, 
as the charts satisfactorily show. ‘There is hence little room to 
doubt that twenty-five fathoms, or 150 feet, may be received 
as the limit in depth of flourishing banks of reef corals. 

It may however be much less, possibly not over half this, on 
the colder border of the coral-reef seas, as, for example, at the 
Hawaian Islands and the atolls northwest of that group. It 
is natural that regions so little favorable for corals on account 
of the temperature should differ in this respect from those in 
the warmer tropics. 

It may be here remarked, that soundings with reference to 
this subject are liable to be incorrectly reported, by persons 
who have not particularly studied living zodphytes. It is of 
the utmost importance, in order that an observation supposed 
to prove the occurrence of living coral should be of any value, 
that fragments should be brought up for examination, in order 
that it may be unequivocally determined whether the corals 
are living or not. Dead corals may make impressions on a 
lead as perfectly as living ones. 

As to the origin of this narrow limit in depth, tempera- 
ture may be one cause through the colder parts of the coral 
seas, it having been proved to be predominant with regard 
to distribution of lfe throughout the extent of the ocean. 
Yet it is not the only cause. The range of temperature 85° 
to 74° gives sufficient heat for the development of the greater 
part of coral-reef species; and yet the temperature at the 
100 foot plane in the middle Pacific is mostly above 74°. 
The chief cause of limitation in depth is the diminished light, 


as pointed out by Prof. T. Fuchs.* 


1 Verh. k. k. geologischen Reichsanstalt, 1882, and Ann. Mag. N. H. Jan. 1883. 


CAUSHS AFFEUTING THE GROWTH OF CORALS. 119 


III. LOCAL CAUSES INFLUENCING DISTRIBUTION. 


Coral making species generally require pure ocean water, 
and they especially abound in the broad inner channels among 
the reefs, within the large lagoons, and in the shallow waters 
outside of the breakers. It is therefore an assertion wide from 
the fact that only small corals grow in the lagoons and chan- 
nels, though true of lagoons and channels of small size, or of 
such parts of the larger channels as immediately adjoin the 
mouths of freshwater streams. 

There are undoubtedly species especially fitted for the open 
ocean; but as peculiar conveniences are required for the col- 
lection of zodphytes outside of the line of breakers, we have 
not the facts necessary for an exact list of such species. From 
the very abundant masses of Astrzeas, Mzandrinas, Porites, 
and Madrepores thrown up by the waves on the exposed reefs, 
it was evident that these genera were well represented in the 
outer seas. In the Paumotus, the single individuals of Porites 
lying upon the shores were at times six or eight feet in diam- 
eter. Around the Duke of York’s Island the bottom was ob- 
served to be covered with small branching and foliaceous 
Montipores, as delicate as any of the species in more protected 
waters. 

Species of the same genera grow in the face of the breakers, 
and some are identical with those that occur also in deeper wa- 
ters. Numerous Astras, Meeandrinas and Madrepores grow 
at the outer edge of the reefs where the waves come tumbling 
in with their full force. There are also many Millepores and 
some Porites and Pocillipores in the same places. But the 
weaker Montipores, excepting incrusting species, are found in 
stiller waters either deep or shallow. 


120 CORALS AND CORAL ISLANDS. 


Again, the same genera occur in the shallow waters of the 
reef inside of the breakers. Astrzeas, Meeandrinas and Pocilli- 
pores are not uncommon, though requiring pure waters. There 
are also Madrepores, some growing even in impure waters. 
One species was the only coral observed in the lagoon of Hon- 
den Island (Paumotus), all others having disappeared, owing 
to its imperfect connection with the sea. Upon the reefs en- 
closing the harbor of Rewa (Viti Lebu), where a large river. 
three hundred yards wide empties, which during freshets en- 
ables vessels at anchor two and a half miles off its mouth to 
dip up fresh water alongside, there is a single porous species 
of Madrepora (M. cribripora), growing here and there in 
patches over a surface of dead coral rock or sand. In similar 
places about other regions, species of Porites are most com- 
mon. In many instances, the living Porites were seen stand- 
ing. six inches above low tide, where they were exposed to sun- 
shine and to rains; and associated with them in such exposed 
situations, there were usually great numbers of Alcyonia and 
Xeniz. The Siderinz endure well exposure to the air. 

The exposure of six inches above low tide, where the tide 
is six feet, as in the Feejees, is of much shorter duration than 
in the Paumotus, where the tide is less than half this amount ; 
and consequently the height of growing coral, as compared 
with low-tide level, varies with the height of the tides. 

Porites also occur in the impure waters adjoining the 
shores; and the massive species in such places commonly 
spread out into flat disks, the top having died from the depo- 
sition of sediment upon it. 

The effects of sediment on growing zoéphytes are strongly 
marked, and may be often perceived when a mingling of fresh 
water alone produces little influence. We have mentioned 
that the Porites are reduced to flattened masses by the lodg- 


CAUSHS AFFECTING THE GROWTH OF CURALS. by | 


ment of sediment. The same takes place with the hemispheres 
of Astrea; and it is not uncommon that in this way large 
areas at top are deprived of life. The other portions still live 
unaffected by the injury thus sustained. Even the Fungia, 
which are broad simple species, are occasionally destroyed over 
a part of the disk through the same cause, and yet the rest re- 
mains alive. It is natural, therefore, that wherever streams or 
currents are moving or transporting sediment, there no corals 
grow ; and for the same reason we find few living zodphytes 
upon sandy or muddy shores. 

The small lagoons, when shut out from the influx of the 
sea, are often rendered too salt for growing zodphytes, in con- 
sequence of evaporation,—a condition of the lagoon of Ender- 
by’s Island. 

They also are liable to become highly heated by the sun, 
which likewise would lead to their depopulation. 

Coral zodphytes sometimes suffer injury from being near 
large fleshy Alcyonia, whose crowded drooping branches lying 
over against them, destroy the polyps and mar the growing 
mass. Again, the dead parts of a zodphyte, though in very many 
cases protected by incrusting nullipores, shells, bryozoans, etc., 
as already explained, in others is weakened by boring shells 
and sponges. Agassiz states, in his paper on the Florida 
Reefs (Coast Survey Report for 1851): ‘‘ Innumerable bor- 
ing animals establish themselves in the lifeless stem, piercing 
holes in all directions into its interior, like so many augurs, 
dissolving its solid connection with the ground, and even pen- 
etrating far into the living portion of these compact communi- 
ties. The number of these |horing animals is quite incredible, 
and they belong to different families of the animal kingdom ; 
among the most active and powerful we would mention the 
date-fish or Lithodomus, several Saxicave, Petricole, Arce, 


129 CORALS AND CORAL ISLANDS. 


and many worms, of which the Serpula is the largest and most 
destructive, inasmuch as it extends constantly through the liv- 
ing part of the coral stems, especially in the Meandrina. On 
the loose basis of a Meandrina, measuring less than two feet 
in diameter, we have counted not less than fifty holes of the 
date-fish—some large enough to admit a finger—besides hun- 
dreds of small ones made by worms. But however efficient 
these boring animals may be in preparing the coral stems for 
decay, there is yet another agent, perhaps still more destruc- 
tive. We allude to the minute boring-sponges, which pene- 
trate them in all directions, until they appear at last com- , 


ae 


| 


pletely rotten through.” 

On the other hand Serpulas and certain kinds of barnacles 
(of the genus Creusia, etc.) penetrate living corals without in- 
jury to them. ‘They attach themselves when young to the sur- 
face of the coral, and finally become imbedded by the increase 
of the zodphyte, without producing any defacement of the sur- 
face, or affecting its growth. Many of these Serpulas grow with 
the same rapidity as the zodphyte, and finally produce a long 
tube, which penetrates deep within the coral mass ; and, when 
alive, they expand a large and brilliant circle or spiral of deli- 
cate rays, making a gorgeous display among the coral polyps. 
Instinct seems to guide these animals in selecting those corals 
which correspond with themselves in rate of growth; and 
there is in general a resemblance between the markings of a 
Creusia and the character of the radiations of the Astrea it in- 
habits. | 

In recapitulation, the three most influential causes of the 
exclusion of reef-forming corals from coasts are the following: 

I. The too low temperature of the waters along shores. 

I]. The too great depth of the waters. 

IIL. The proximity of the mouths of rivers, on account of 


RATE OF GROWTH OF CORALS. 123 


which sediment is distributed along the coast adjoining and 
over the sea bottom. 


IV. RATE OF GROWTH OF CORALS. 


The rate of growth of coral is a subject but little under- 
stood. We do not refer here to the progress of a reef in for- 
mation, which is another question complicated by many co-op- 
erating causes; but simply to the rapidity with which partic- 
ular living species increase in size. There is no doubt that 
the rate is different for different species. It is moreover prob- 
able that it corresponds with the rate of growth of other al- 
lied polyps that do not secrete lime. The rate of growth of 
Actinie might give us an approximation to the rate of growth 
in coral animals of like size and general character; for the ad- 
ditional function of secreting lime would not necessarily re- 
tard the maturing of the polyp; and from the rate of growth 
of the same animals in the young state, we might perhaps 
draw some inferences as to the rate in polyps of corresponding 
size. But no satisfactory observations on this point have yet 
been made. 

Although the rapidity is undoubtedly far less than was 
formerly reported, the following facts from different sources 
seem to show that the rate is greater than has been of late be- 
lieved. Mr. Darwin, citing from a manuscript by Dr. Allan, 
of Forres, some experiments made on the east coast of Mada- 
gascar, states that, in December, 1830, twenty corals were 
weighed, and then placed by him apart on a sandbank, in three 
feet water (low tide), andin the July following, each had nearly 
reached the surface and was quite immovable; and some had 
grown over the others. Mr. Darwin mentions also a state- 
ment made to him by Lieut Wellstead, that ‘in the Persian 


124 CORALS AND CORAL ISLANDS. 


Gulf a ship had her copper bottom encrusted in the course of 
twenty months, with a layer of coral two feet thick,—evi- 
dently to be accepted hesitatingly. He also speaks of a chan- 
nel in the lagoon of Keeling atoll having been stopped up in 
less than ten years; and of the natives of the Maldives find- 
ing it necessary occasionally to root out, as they express it, 
coral knolls from their harbors. 

Mr. Stutchbury describes a specimen consisting of a spe- 
cies of oyster whose age could not be over two years, encrust- 
ed by an Agaricia weighing two pounds nine ounces; but he 
does not state whether the shell was that of a living oyster 
or not. 

Dr. D. F. Weinland states that on Hayti, in a small coral 
basin between the town of Corail and the island Caymites, 
never disturbed by vessels on account of the small depth of 
water, he observed several branches of the Madrepora cervi- 
cornis projecting above the surface of the water from three to 
five inches, all of which, down to the water level, were dead, 
as a result evidently of exposure to the air. This was in the 
month of June. He adds that all along the north shore of 
Hayti, the water level is from four to six feet higher in the 
winter season than during summer; and suggests that the 
growth of three to five inches, above referred to, might. have 
been made during the three winter months. 

Duchassaing (in L’Institut, 1846, p. 117) observes that in 
two months some large individuals of Madrepora prolifera 
which he broke away, were restored to their original size. 
More definite and valuable is the observation of Mr. L. F. de 
Pourtales, that a specimen of Meandrina labyrinthica, meas: 
uring a foot in diameter, and four inches thick in the most 
convex part, was taken from a block of concrete at Fort Jet: 
terson, Tortugas, which had been in the water only twenty 


RATE OF GROWTH OF CORALS. 125 


years. Again, Major E. B. Hunt mentions, in the American 
Journal of Science for 1863, the fact of the growth of a Mean- 
drina at Key West, Florida, to a radius of six inches in twelve 
years, showing an average upward increase in this hemispherical 
coral of half an inch a year, if, as is evidently implied, this 
radius was a vertical radius. Major Hunt deposited speci- 
mens of corals of his collection near Fort Taylor, Key West, 
in the Yale College Museum, and three of these are labelled 
by him as having grown to their present size between the 
years 1846 and 1860, or in fourteen years. ‘Two are speci- 
mens of Oculina diffusa; one is a clump four inches high 
and eight broad; and the other has about the same height. 
The weight of the first of these clumps is forty-four ounces. 
The rate of four inches in fourteen years would be equal to 
about 34 twelfths of an inch a year in height, or three and one- 
seventh ounces a year of solid coral. The other specimen is 
of the Meandrina clivosa V.; it has a height of two and a 
quarter inches and a breadth of seven and a half inches. This 
is equivalent to about a sixth of an inch of upward growth 
in fourteen years. ‘The specimen weighs about eighteen ounces. 
It is not certain that with either of these specimens the germs 
commenced to grow the first year of this interval, and hence 
there is much doubt with regard to these calculations. 

The following observations are from a paper read by 
Prof. Verrill before the Boston Society of Natural History 
in 1862. The wreck of a vessel, supposed to have been the 
British frigate Severn, lost in 1793 near “Silver Bay,” off 
Turk’s Islands, is covered with growing corals. It lies (accord- 
ing to the journal of Mr. J. A. Whipple, by whom specimens 
were collected in 1857) in about four fathoms of water. One 
of the specimens was a mass of the species Orbicella annula- 
ris, shaped somewhat like a hat; it is attached to the top of a 


126 CORALS AND CORAL ISLANDS. 


bell and spreads outward on all sides. The thickness of the 
coral at the centre is about eight inches, and the breadth fit: 
teen. Another specimen consisted of an olive jar and glass 
decanters cemented together by a mass, of like size, of the same 
species of coral. The interval since the wrecking of the ves- 
sel, to 1857, was sixty-four years, and if the corals commenced 
their growth immediately after the wreck the increase of this 
species of coral is very slow. 

The journal of Mr. Whipple, in the library of the same 
society, contains the records of his observations on the spot, 
and the efforts made to remove the corals in order to examine 
the wreck. The following are a few extracts made from it by 
Prof. Verrill: 

April 21, 1857.—Moored our boat over the remains of a 
large wreck, * * its depth being from three to ten fath- 
oms. I made the first descent in the armor. I found the bot- 
tom very uneven and covered with the remains of a man-of- 
war, what appeared to be the bow lying in a gulch, with the 
shanks of three large anchors, the palm of only one of which 
projected out of the coral rock. 

April 22.—Made a second descent and commenced exam- 
ining in six fathoms of water on what appeared to be mid- 
ships. All astern of this is thick branching coral (Madrepora), 
and it must have made very fast, the branches being twelve 
inches in diameter and sixteen feet in height. ‘To look among 
it from the bottom reminds one of a thick forest of a heavy 
srowth of timber. * * * ‘This branched coral appears to 
grow where there is but very little iron, as I could see no guns 
or shot around its roots. Commenced examining the cannon 
with hammer and chisel. * * * Near these cannon, which 
must have been near the forward part of the ship, I com- 
menced to work on a clear space between the cannon. After 


RATH OF GROWTH OF CORALS. L237 


breaking three inches of coral crust I found the collar bone 
of a man, a brass regulating screw belonging to a quadrant, 


sian eee fale 


and some large lead bullets. The magazine must 


be under the branch-coral, which has been sixty-four years 
meawing, ~ ~ * 

Here we have a height of sixteen feet in a Madrepora 
attained in sixty-four years, or at the rate of three inches a 
year. Observations of Prof. Joseph Le Conte on Madre- 
pora growths at the Tortugas in 1851 (American Journal 
of Science, 1875) lead to a rate of 3} inches a year. 

Observations on the rate of growth of different species 
might easily be made by those residing in coral seas, either in 
the manner adopted by Mr. Allan (placing the specimens on 
a platform which could be raised for examination from time 
to time—say every five years), or by placing marks upon par- 
ticular species where they are immovably fixed to the bottom. 
By inserting slender glass pins a certain distance from the sum- 
mit of a Madrepore, its growth might be accurately measured 
from month to month. Two such pins in the surface of an 
Astra, would in the same manner, by the enlarging distance 
between, show the rate of increase in the circumference of 
the hemisphere; or if four were placed so as to enclose an 
area, and the number of polyps counted, the numerical in- 
crease of polyps resulting from budding, might be ascer- 
tained. If specimens are selected, as done by Mr. Allan, it is 
important that they should be placed where other corals are 
growing in luxuriance, so as to be sure that there are no dele- 
terious influences to retard growth. It is to be hoped that 
some of the foreign residents at the Sandwich, Society, Samo- 
an or Feejee Islands will take this subject in hand. There arc 
also many parts of the West Indies where these investiga- 
tions might be conveniently made. 


128 CORALS AND CORAL ISLANDS. 


CHAPTER II. 
STRUCTURE OF CORAL REEFS AND ISLANDS. 


Corat reefs and coral islands are structures of the same 
kind under somewhat different conditions. They are made 
in the same seas, by the same means; in fact, a coral island 
has in all cases been a coral reef through a large part of its 


history, and is so still over much of its area. The terms how- — 


ever are not synonymous. Coral islands are reefs that stand 


isolated in the ocean, away from other lands, whether now 


raised only to the water’s edge and half submerged, or covered 
with vegetation ; while the term coral reef, although used for 
reefs of coral in general, is more especially applied to those 
which occur along the shores of high islands and continents. 
There are peculiarities in each making it convenient to describe 
them separately. 


I. CORAL REEFS. 


IL GENERAL FEATURES. 


Coral reefs are bans of coral rock built upon the sea-bot- 
tom about the shores of tropical lands. In the Pacitic, these 
lands, with the exception of New Caledonia and others of 
large size to the westward, are islands of volcanic or igneous 
rocks. and they often rise to mountain heights. The coral 
reefs which skirt their shores are ordinarily wholly submerged 
at high tide; but, at the ebb, they commonly present to view a 
broad, flat, bare surface of rock, just above the water level, 


os 


STRUCTURE OF CORAL REEFS. 129 


strongly contrasting with the steep slopes of the encircled 
island. 

Nearing in a vessel a coral-bound coast, the first sign of the 
reef, when the tide is well in, is a line of heavy breakers, per- 
haps miles in length, off a great distance from the land. On 
closer view, some spots of bare reef may be distinguished as 
the waves retreat for another plunge; but the next moment 
all again is an interminable line of careering waters. Happy 
for the cruiser in untried reef-regions, if the surging waves con- 
tinue to mark the line of reef; for a treacherous quiet some- 
times intervenes, which seems to be evidence of deep waters 
ahead, and the unsuspecting craft dashes onward ; but soon it 
is grinding over the coral masses, then thumping heavily at 
short intervals, and, in a few moments more, is landed helpless 
on the coral reef. The heavier billows as they roll by a vessel 
in such a plight—the author’s experience attesting—have a 
way of lifting it and then letting it drop with all its 
weight against the bottom, and hence, unless prompt escape is 
in some way secured, the assaulting waves gain speedy posses- 
sion, and soon after make complete the work of destruction. 
At low tide the breakers often cease, or nearly so. But the 
reef for the most part, is then in full view, and, with a good 
lookout aloft, favorable winds, and plenty of daylight, navi- 
gation is comparatively safe. 

Some idea of the features of a tropical island thus bor- 
dered, may be derived from the following sketch. The reef 
to the right is observed to fringe the shore, making a simple 
broad platform, as an extension, apparently, of the dry land. 
To the left there is the same coral platform at the surface, but 
it is divided by a channel into an inner and an outer reef—a 
Jringing and a barrier reef, as these two parts are called. At 


a single place the sea is faced by a cliff; and here, owing to 
9 


130 CORALS AND CORAL ISLANDS. 


the boldness of the shores and depth of waters, the reef is 
wanting. The barrier reef at one point has a passage through 
it, which is an opening to a harbor; and many such harbors 
exist about coral-girt islands. 


























































































































































































































































































































































































































































































































HIGH ISLAND WITH BARRIER AND FRINGING REEFS. 


While some islands have only narrow fringing reefs, others 
are almost or quite surrounded by the distant barrier, which 
stands off like an artificial mole to protect the land from an 
encroaching ocean. ‘The barrier is occasionally ten or fifteen 
miles from the land, and encloses not only one, but at times 
several, high islands. From reefs of this large size, there are 
all possible variations down to the simple fringing platform. 

The inner channel is sometimes barely deep enough at low - 
tide for canoes, or for long distances may be wanting entirely. 
“hen again, it is a narrow intricate passage, obstructed by 

1olls or patches of coral, rendering the navigation dangerous. 

gain, it is for miles in length an open sea, in which ships 
find room to beat against a head wind with a depth of ten, 
twenty, or even thirty fathoms. Yet hidden reefs make caution 
necessary. Patches of growing corals, from a few square feet 
to many square miles in extent, are met with over the broad 
area enclosed by these distant barriers. 

These varieties of form and position are well exemplified 
in a single group of islands—the Feejees; and the reader is 
referred to the chart of this Archipelago at the close of this 


volume. x 


STRUCTURE OF CORAL REEFS. 131 


Near the middle of the chart is the island Goro ; its shores, 
excepting the western, are bordered by a fringing reef. The 
island Angau, south of Goro, is encircled by a coral breakwa- 
ter, which on the southern and western sides runs far from the 
shores, and is a proper barrier reef, while on the eastern side, 
the same reef is attached to the coast and is a fringing reef. 
From these examples we perceive the close relation of barrier 
and fringing reefs. While a reef is sometimes quite encircling, 
in other instances it is interrupted, or wholly wanting, along 
certain shores ; and occasionally it may be confined to asingle 
point of an island. 

Above Angau lies Naira: ; although a smaller island than 
Angau, the barrier reef is of greater extent, and stretches off 
far from the shores. *To the eastward of Nairai are Vatu 
- Rera, Chichia, and Naiau, other examples of islands fringed 
around with narrow reefs. Lakemba, a little more to the 
southward, is also encircled with coral ; but on the east side 
the reef is a distant barrier. In Azva, immediately south of 
Lakemba, the same structure is exemplified; but the coral 
ring is singularly large for the little spots of land it encloses. 
The Argo Reef, east of Lakemba, is a still larger barrier, en- 
circling two points of rock called Bacon’s Isles. It is actually 
a large lagoon island, twenty miles long, with some coral islets 
in the lagoon, and two of basaltic constitution, of which the 
largest is only a mile in diameter. Aiva and Lakemba are in 
fact other lagoon islands, in which the rocky islands of the in- 
terior bear a larger proportion to the whole area. The same 
view is further illustrated by comparing the Argo reef with 
Nairai, Angau, or Moala: these cases differ only in the great- 
er or less distance of the reef from the shores and the extent 
of the enclosed land. 

« Passing to the large islands Vanua Levu and Viti Levu, 


132 CORALS AND CORAL ISLANDS. 


we observe the same peculiarities illustrated on a much grand- 
er scale. Along the southern shores of Viti Levu, the coral 
reef lies close against the coast; and the same is seen on the 
east side and north extremity of Vanua Levu. But on the 
west side of these islands, this reef stretches far off from the 
land, and in some parts is even twenty-five miles distant, with 


a broad sea within. ‘This sea, however, is obstructed by reefs, 


and along the shores there are proper fringing reefs. 

The forms of encircling reefs depend evidently to a great 
extent on that of the land they enclose. That this is the case 
even in the Argo reef, and such other examples as offer now 
but a single rock above the surface of the enclosed lagoon, we 
shall endeavor to make apparent, if not already so, when the 
cause of the forms of coral islands is under discussion. Yet it 
is also evident that this correspondence is not exact, for many 
parts of the shores, and sometimes more than half the coasts, 
may be exposed to the sea, while other portions are protected 
by a wide barrier. 

In recapitulation, we remark, that reefs around islands may 
be (1) entirely encircling ; or they may be (2) confined to a larg- 
er or a smaller portion of the coast, either continuous or inter- 
rupted; they may (3) constitute throughout a distant barrier ; 
or (4) the reef may be fringing in one part and a barrier in 
another; or (5) it may be fringing alone: the barrier may be 
(6) at a great distance from the shores, with a wide sea within, 
or (7) it may so unite to the fringing reef that the channel be- 
tween will hardly float a canoe. These points are sustained 
by all reef regions. 

It is to be noted that the fringing and barrier reefs here 
pointed out are not the whole of the coral reef; they are only 
the portions that have been built up to the water's level. Be- 
tween them, and also outside of all, there are the submerged 


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STRUCTURE OF CORAL REEFS. 135 


coral banks which are continuous with the higher portions, and 
all together make up the coral reef-ground of an island. 

A wide difference in the extent of reef-grounds, follows 
from the above-mentioned facts. On some coasts there are only 
scattered groups of corals, or rising knolls, or mere points of 
emerged coral rock ; but again, as for example, west of the two 
large Feejee Islands, there may be three thousand square miles 
of continuous reef-ground, occupied with coral patches and in- 
termediate channels or seas. ‘The enclosing barrier off Vanua 
Levu alone is more than one hundred miles long. The Ex- 
ploring Isles, in the eastern part of the Feejee group, have a 
barrier eighty miles in circuit. New Caledonia has a reef 
along its whole western shores, a distance of two hundred and 
fifty miles, and it extends one hundred and fifty miles farther 
north, adding this much to the length of the island. The 
great Australian barrier forms a broken line, twelve hundred 
and fifty miles in length, lying off the coast from the Northern 
Cape to the tropical circle. 

In the Louisiade Archipelago, Plate VII., the area within 
the great reef, one hundred and twenty-five miles long, is 
five sixths water, with depths of ten to two hundred feet ; 
and the westernmost island is an atoll. 

In the further description of reef-grounds, we note: 

1. Outer reefs, or reets formed from the growth of corals 
exposed to the open seas. Of this character are all proper 
barrier reefs, and such fringing reefs as are unprotected by a 
barrier. 

2. Inner reefs, or reefs formed in quiet water between a 
barrier and the shores of an island. 

3. Channels, or seas within barriers, which may receive de- 
tritus either from the reefs, or from the shores, or from both of 


these sources combined 


136 CORALS AND CORAL ISLANDS. 


4, Beach and Drift formations, produced by coral accu- 
mulations on the shores through the action of the sea and 
winds, 

The outer and inner reefs, channels, and beaches, act each 
their part in producing the coral formations in progress about 
islands. 


Il. OUTER REEFS. 


The barrier and other outer reefs are always submerged at 
high tide, except where elevated at surface by accumulations 
of beach sands. The level is generally that of about one third 
tide. The coral rock is built up by the agencies at work to 
this level, and hence the existence of the broad plattorm-like 
top of the barrier. The surface is however not even, for there 
are many pools of water over it, even at the lowest tides, espe- 
cially toward its outer limits, where corals of various kinds are 
crowing luxuriantly, with fit associates of shells, star-fishes, 
echini, holothurias with their large flower-bearing heads, 
sponges, corallines and sea-weeds, making scenes of rare beauty. 
The growing corals are, however, most abundant along the outer 
margin of the reef, and in the adjoming shallow seas. Here 
they grow in profusion ; but yet the eager lover of coral land- 
scapes will be often disappointed by finding among the crowd- 
ed plantations, extensive areas of coral sand. 

The outer margin of the reef receives the plunging waves, 
and under this action, and the consequent unequal growth of 
the corals, the outline is very irregular, being often deeply cut 
into, and hence having sometimes long channels that give en- 
trance to the surging tide, and to the currents that flow back 
in preparation for the next breaker. From it, seaward, the 
depth of water usually sinks off rapidly from three to six fath- 
oms, and then falls away more gradually for many rods, or it 


STRUCTURE OF CORAL REEFS. 137 


may be some hundreds of yards ; over the bottom in these shal. 
low waters are spread out the coral plantations, down to a 
depth of 80 to 150 feet. Finally there is a rather abrupt de 
scent to depths beyond the reach of an ordinary sounding-lead. 
The great difference in the rapidity with which the water deep- 
ens depends chiefly on the varied character of submarine 
slopes. Shallow waters may extend out for miles, especially 
off the prominent points or angles; but it is more common to 
meet with the opposite extreme—great depths within a few 
hundred feet. 

The outer reef or coral platform is generally a little the 
highest at its seaward margin, owing partly to the growth of 
ordinary corals and other species on this part, and also to the 
accumulations which naturally would there be piled up by 
the waves and become cemented. ‘This part is therefore first 
laid bare by the retreating tide; and though a tempting place 
for a ramble, it is often a dangerous place on account of the 
heavy breakers. There is not only greater height, but often 
also a remarkably smooth surface to the reef-rock, looking as 
if water-worn, and frequently a blotching of the rock with va- 
rious shades of pink and purple. These colors and the smooth- 
ness, as observed by Chamisso, are due to incrusting Nulli- 
pores; and to the same calcareous sea-weeds, as Darwin first 
observed, is often owing the increased height. The material 
of the incrusting plant is more solid than ordinary coral, for it 
is without a pore; and layer is added to layer until it has con- 
siderable thickness. It is thus an important protection to the 
reef against the wash of the waters. 

Darwin states that on Keeling Island, the Nullipore bed 
has a thickness of two or three feet and a breadth of twenty 
feet. Nullipores are abundant on the Paumotu reefs. Still, 
they are not essential to the formation or protection of an 


138 CORALS AND CORAL ISLANDS. 


outer reef, and are not always present; the outer margin is 
higher than the rest of the reef when they are absent. 

The Nullipores are not alone on this outer edge, for there 
are always sprigs of Madrepores, small Astrzeas, and some oth- 
er corals, lodged in the cavities, with many Echini, star-fishes 
and sea-anemones, besides barnacles and serpulas; and fish of 
many colors dart in and out of the numerous recesses. 

Outer reefs are far more lable than the inner to become 
covered with accumulations of coral fragments and sand 
through the force and inward movement of the waves. The 
debris gathered up by the waters finds a lodgment some dis- 
tance back from the margin—it may be one or two hundred 
fect, or as many yards, and gradually increases, until in many 
instances dry land is formed, and an islet covered with vegeta- 
tion appears. Such effects are confined chiefly to the reef on 
the sides open to the prevailing wind, and the final result, a 
green islet, is not of common occurrence. But occasionally, 
the reef for miles has become changed from the coral bank, 
bare at low or middle tide, to habitable land, and makes liter- 
ally, as at Bolabola, a green belt to the island of volcanic rocks 
and lofty hills within. The causes and the result are much the 
same as in a coral island, and the steps in the process are 
more particularly described beyond where treating of atolls. 

The rock of the outer reef, wherever broken, exhibits usu- 
ally a compact texture. In some parts it consists of coral 
fragments, rounded or angular, of quite large size, firmly ce- 
mented. Other portions are a finer coral breccia or conglom- 
erate. Still others, more common, are solid white limestones, 
as impalpable and homogeneous in texture as the old limestones 
of our continents. There are also other regions where the 
corals in the rock retain the original position of growth. But 
the rock in general consists of the debris of the coral fields, 


STRUCTURE OF CORAL REEFS. 139 


consolidated by a calcareous cement; and the great abundance 
of the finer variety of rock indicates that much of it has orig- 
inated from coral sand or mud. Wherever broken, it usually 
presents the character here described, a texture indicating a 
detrital or conglomeritic origin. Such a reef-rock is formed 
in the midst of the waves; and to this fact it owes many of its 
peculiarities. Reef-rocks made of corals in the position of 
erowth are formed about the outer reefs wherever the corals 
grow undisturbed. 

Besides corals, the shells of the seas contribute to it, and it 
sometimes contains them as fossils, along with bones of fishes, 
exuvia of crabs, spines and fragments of Echini, Orbitolites 
(disk-shaped foraminifers), the tubes of Serpule or sea-worms, 
and other remains of organic life inhabiting reef-grounds. 


, 11. FORMATIONS IN THE SEA OUTSIDE OF THE BARRIER REEFS. 


While barrier reefs are mostly made up of coarse coral ma- 
terial, owing to the rough action of the waves, the region im- 
mediately outside of the breakers, where of much width, is, to a 
depth of 50 to 150 feet, one of growing patches of coral and 
extended surfaces of coral sands. 

Isolated islets of reef-rock are not however of common oc- 
currence in the middle Pacific, though occurring in large groups 
like the Feejees. They are most likely to occur where there 
are great regions of shallow water extending outward from the 
barrier, and where the tides are not heavy or there is partial pro- 
tection from them. In some seas, such isolated patches are shaped 
somewhat like a great mushroom—having a narrow trunk 
or column below, supporting a broad shelf of reef above. Mr. 
J. A. Whipple, in his Journal, referred to on page 126, figures 
and describes one of these ‘coral heads” standing in water fit- 
ty feet deep, near Turks Island. Its trunk, which made up 


140 CORALS AND CORAL ISLANDS. 


two thirds of its height (or of the fifty feet), was only fifteen 
feet in diameter along its upper half; and it supported above a 
great tabular mass one hundred feet in diameter, whose top was 
bare at low tide. ‘The tide at this place is but two feet, and 
this is favorable to the preservation of such top-heavy struc- 
tures. In many places, he says, these tops have joined together, 
leaving arches between them ; and in some parts of the reef-re- 
gion such united coral-heads cover acres in extent, being joined 
together above and supported by their pillars. A case is re- 
ported of a whale having gone through one of these under 
passages after being struck with a harpoon. Mr. Whipple 
also states that there are cavernous recesses in some of these 
heads, some that are 200 to 300 feet across; and ‘“‘ when there 
is a heavy swell on, the water is one entire sheet of white foam, 
caused by its being forced through them and the air entering 
as the heavy sea recedes from them.” 

































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































THE LIXO CORAL REEF, ABROLHOS. 


Professor C. F. Hartt, in his ‘‘ Geology, ete., of Brazil” 
(1870), describes very similar coral-heads in his account of the 
reefs of the Abrolhos, and represents a scene of coral-head tops 
in a sketch, of which the preceding is a copy. Professor Hartt 
speaks of it as giving simply a general view of the region with- 


STRUCTURE OF CORAL REEFS. eI 


out any attempt at accuracy of position. The patches of reef 
in the view are of this coral-head kind, though not all as slen- 
derly supported as that above described. A vessel is represent- 
ed passing through a passage between two of them. Prof. 
Hartt, after describing the fringing reefs of the Abrolhos, gives 
the following account of the outside coral formations (p. 199). 
“Corals grow over the bottom in small patches, 7n the open sea, 
and, without spreading much, often rise to a height of forty or 
fifty or more feet, like towers, and sometimes attain the level 
of low water, forming what are called on the Brazilian coast 
chapeiroes (signifying big hats). At the top these are usually 
very irregular, and sometimes spread out like mushrooms, or, 
as the fishermen say, like umbrellas. Some of these chapei- 
rdes are only a few feet in diameter. A few miles to the east- 
ward of the Abrolhos is an area, with a length of nine to ten 
and in some places a breadth of four miles, over which these 
structures grow abundantly, forming the well known Parcel 
dos Abrolhos, on which so many vessels have been wrecked.” 
‘“‘ Among these chapeirdes I measured a depth of sixteen to 
twenty metres, and once, while becalmed, I found twenty me- 
tres alongside of one and three metres on top. They are 
rarely laid bare by the tide. They do not coalesce here to 
form large reefs as they do to the west of the islands. * * * 
Sometimes vessels striking heavily on small chapeirdes, break 
them off and escape without injury, as has been remarked by 
Mouchez. At other times a vessel may run upon one and stick 
fast by the middle of the keel, to the amazement of the cap- 
tain, who finds deep water all around, the vessel being perched 
on the chapeirges like a weather-cock on the top of a tower.” 
“In the northern part of the Parcel the chapeirées so close- 
ly unite as to form an immense reef, which has grown upward 
to a level a little above low water, and is quite uncovered at 


142 CORALS AND CORAL ISLANDS. 


low tide.” “The northeastern part of the reef is called the, 


Recife do Lixo, that is, Reef of the /zxo, a shark-like ray which is 
furnished with large crushing teeth and frequents the reef in 
search of shell-fish.” 

The rock of the submerged coral-heads is but a loose ag- 
gregation of corals in the position of growth, except probably, 
in their lower portion, where the open spaces may be filled 
with sand and fragments and all cemented together. 

The deposits of sand or coral mud over the bottom of the 
seas outside of barrier reefs are sometimes of great extent. 
These sands are the fine detritus which the return flow of the 
breaker bears seaward ; and, in still deeper water, the deposits 
should be mainly of the finest calcareous sand or mud—fit ma- 
terial for impalpable compact limestones. The waters outside 
of the reef, especially when moved by heavy tidal currents or 
storms, are often milky with the coral sand; and while the 
coarser sand is dropped near the shores, the finer may be 
. carried for miles and distributed far out to sea. As Major 
Hunt, in his observations on the Florida Reefs remarks, this 
‘white water” is one of the signs of proximity to a coral reef. 
After storms, the white coral material subsides and the waters 
become clear again. 

Mr. Jukes, who made special examinations of the Australi- 
an reef region, and others in that vicinity, in H. M.S. Fly, 
states that in the deeper waters outside of the great barrier, 
‘and in all the neighboring East India seas, from Torres 
Straits, north of Austraiia, to the Straits of Malacca, wher- 
ever the bottom was brought up by the lead, it proved to 
be a very fine-grained, impalpable, pale olive-green mud, 
wholly soluble in dilute hydrochloric acid, and therefore essen- 
tially carbonate of lime. The substance, when dried, looked 
much like chalk, excepting in its greener tinge. How far this 


STRUCTURE OF CORAL REEFS. 143 


calcareous matter may be due to foraminifers, rather than cor- 
als, is not known.” 

Since the tidal waves on any coast that is gradually shal- 
lowing have a landward propelling power, the coral sands are 
mostly gathered about the reef, and generally are not to any 
great extent lost in the depths of the ocean. The great ocean- 
ic currents, like that of the Gulf stream, might bear away the 
lighter material for long distances, if it swept with full strength 
over the shore reefs; but it is generally true that such cur- 
rents are little felt close in shore. Notwithstanding the prox- 
imity of the Florida reets, and the strength of the Gulf stream 
in the channel between the Keys and Florida, the adjoining 
sea-bottom consists mainly of common inud, with relics of deep 
water life, and only sparingly of coral débris. According to 
Mr. L. F. de Pourtales, between twelve fathoms and one 
hundred, in the Florida channel, outside of the reef, coral frag- 
ments occur, but are rare; dead specimens of Cladocora and 
Oculina occur to a depth of about 50 fathoms. But on the 
other side of the channel, ‘‘along the Salt Key Bank, dead 
corals were dredged up in 315 fathoms ; but this is at the foot 
of a very steep slope washed by the edge of the Gulf stream ; 
which is much better defined here than on the Florida side.” 
The bottom, in the Florida channel, of 100 fathoms, is a rocky 
plateau, and outside of 200 fathoms, a mud full of foraminifers, 
Globigerina mud, as it is called from the species characterizing 
it; and yet this channel is situated beneath the Gulf stream and 
close by the Florida reefs. The facts seem to show that in most 
regions the reefs contribute little calcareous matter to the deep 
ocean. ‘This may be otherwise over the bottom, of compara- 
tively little depth, of a great Archipelago like that of the East 
Indies. 


144 CORALS AND CORAL ISLANDS. 


IV. INNER REEFS. 


In the still waters of the inner channels or lagoons, when 
of large extent, we find corals growing in their greatest per. 
fection, and the richest views are presented to the explorer of 
coral scenery. There are many regions—in the Feejees, ex- 
amples are common—where a remote barrier encloses as pure 
a sea as the ocean beyond; and the greatest agitation is only 
such as the wind may excite on a narrow lake or channel. 
This condition gives rise to some important peculiarities of 
structure in the inner reefs, in which the inner margin of the 
barrier reef participates. 

In the general appearance of the surface, the inner gener- 
ally much resemble the outer reefs. They are nearly flat, and, 
though mostly bare of life, and much covered with coral sand, 
there are seldom any large accumulations of coral débris. The 
margin is generally less abrupt; yet there is every variety of 
slope, from the gradually inclined bed of corals to the bluff de- 
clivity with its clinging clumps. In different parts, there are 
many: portions still under water at the lowest tides; and here 
(as well as upon the outer banks) fine fishing sport is afforded 
the natives, who wade out at ebb tide with spears, pronged 
sticks, and nets, to supply themselves with food. The lover of 
the marvellous may find abundant gratification by joining in 
such a ramble; for besides living corals, there are myriads of 
other beings which science alone has named, of various beauti- 
ful forms and colors, as becomes the inhabitants of a coral world. 

Between the large reefs, which spread a broad surface, at 
the water’s edge, of lifeless coral rock, sometimes of great ex- 
tent, there are other patches, still submerged, that are cov- 
ered with growing corals throughout. ‘They are of different 
elevations under the water’s surface; and though at times but 


STRUCTURE OF CORAL REEFS. 145 


a few yards in breadth, there is often alongside of them a 
depth of many fathoms. The mushroom shape described 
above is common among them; and a ship striking one with 
her keel may crush it and glide on. More frequently, they 
are at bottom like the solid reef above described, and the con- 
test is more likely to be fatal to the vessel than to the coral 
patch. Ina passage between two reefs near Tongatabu, called 
the Astrolabe channel, the sloop-of-war Vincennes ran on a 
coral patch, which had been laid down as a reef. It stopped 
the ship for a moment, but broke away under her; and in the 
survey of the passage afterward, says Captain Wilkes, “no shoal 
was found in the place where the ship had struck, and we had the 
satisfaction of knowing that we had destroyed it without injury 
to the vessel.” Corals grow over these patches, as in the shal- 
low waters about other reefs ; and, as elsewhere, there are deep 
cavities among the congregated corals, in which a lead will some- 
times sink to a depth of many feet, or even fathoms. These 
holes about growing reefs often give much annoyance to the 
boat which may venture to’anchor upon them; and in many 
an instance diving is found to be the only resource left for free 
ing the foul anchor. 

The margins of the reefs in and about the inner channels 
are often luxuriant with magnificent corals quite to the edge, 
so that while the reef is elsewhere solid rock to its very top, 
here at the margin it is alive and may be said literally to be 
growing. 

The rock of the inner reefs seldom consists of rolled or 
broken fragments of coral like a large part of that of the 
outer reef. It is often made of dead corals, standing to a 
great extent as they grew; yet it is generally compact and 
firm in texture. The cavities among the branches and masses 


gradually become filled with coral sand, and the whole is 
10 


146 CORALS AND CORAL ISLANDS. 


finally cemented and so made solid. At Tongatabu and 
among the Feejee Islands, reefs thus formed of corals standing 
in their growing positions are common. ‘Though now mere 
dead rock, and exceedingly firm and compact, the limits of 
the several constituent coral masses may be distinctly made 
out. Some individual specimens of Porites in the rock of the 
inner reef of Tongatabu are twenty-five feet in diameter ; and 
Astras and Meeandrinas, both there and in the Feejees, meas- 
ure twelve to fifteen feet. These corals, when growing be- 
neath the water, form, as has been stated, solid hemispheres, 
or rounded hillocks; but on reaching the surface, the top dies, 
and enlargement takes place only on the sides; and in this 
manner the hemisphere is finally changed to a broad cylinder 
with a flat top. This was the condition of the Astraas and 
Porites in the reef-rock referred to. Such a platform looks 
like a Cyclopean pavement, except that the calcareous ce- 
menting material, fillmg im between the huge masses, is more 
solid than in any work of art: it even exceeds in compactness 
the corals themselves. Other portions of reefs consist of 
branching corals, with the intervals filled in by sand and small 
fragments; for even in the stiller waters fragments are to some 
extent produced. <A rock of this kind is often used for build- 
ings and for walls on the island of Oahu. It consists mainly 
of Porites, and in many parts is still cavernous, or but imper- 
fectly cemented. 

There is also to be found about inner reefs, over large 
areas, the solid white limestone already described, showing 
internally no evidence of its coral origin, and containing rarely 
a shell or other imbedded fossil. It is a result of the consoli- 
dation of the fine coral sand or mud that is made and accu- 
mulated through the action of the light waves that work over 
the inner reefs. It has been said that large regions of barren 


\ 
\ 
} 





STRUCTURE OF CORAL REEFS. 147 


sands or mud occur among the patches of growing corals, and 
these would give origin to this compact limestone. 

The formation of the inner reefs goes on at a less rapid 
rate than that of the outer, because the process depends on the 
growth of the corals with comparatively little aid from the 
action of the waves. Moreover, as is explained more par- 
ticularly in another place, impure or fresh waters and cur- 
rents often operate to destroy the living corals or retard their 
progress. 

Owing to the last mentioned cause, the inner reefs are not 
usually joined directly to the beach. They stand off a little, 
separated by an interval of shallow water. At Mathuata, in 
the Feejees, however, the reef extends quite up; and it is 
the more remarkable as the coast is flat, the site of a Feejee 
village, and a mile or two back stands a high bluff. On an 
island off this part of Vanua Lebu there is another exam- 
ple of this fact, and many more might be cited. In such 
cases, however, there is evidence that the shores upon which 
the corals grew were bare rocks, instead of moving beach- 
sands. 

From these descriptions it appears that the main distinc- 
tion between the inner and outer reefs consists in the less frag- 
mentary character of the rock in the former case, the less fre- 
quent accumulations of débris on their upper surface, and the 
more varied features and slopes of the margin. Moreover, 
the Nullipores, which seem to flourish best in the breakers, 
are here but sparingly met with. 

The variety of coral zodphytes is also greater in the stiller 
waters, when these have great breadth, and there are species 


peculiar to the different regions. 


148 «~ \ CORALS AND CORAL ISLANDS. | 


V. CHANNELS AMONG REEFS. 


To complete this review of the general appearance and 
constitution of reef formations, it remains to add some partic- 
lars respecting the channels which intervene between coral 
patches, or separate them from the shores of an island, and 
also to describe the coral accumulations forming beaches. 

The reef of Australia has been instanced as affording an 
example of one of the larger reef-channels, varying from twenty 
to sixty miles in width, and as many fathoms in depth. Its 
average distance from the land is twenty to thirty miles, and 
the ordinary depth ten to twenty-five fathoms; but toward the 
southern end, where the channel is widest, the depth exceeds 
sixty fathoms. ‘‘ The new Caledonia barrier reefs, 400 miles in 
length,” says Darwin, “ seldom approach within eight miles of 
the shore.” The reefs west of the large Feejee Islands are 
another remarkable example, the reef-grounds being in some 
parts twenty-five miles wide, and the waters within the bar- 
rier, where sounded, twelve to forty fathoms in depth. ‘The 
barrier in this instance may be from a few hundred yards to 
half a mile in width; and some of the inner patches are of 
the same extent; but by far the larger part of the reef-ground 
is covered with deep waters, mostly blue like the ocean, and 
as clear and pure. In the course of the cruise of the Wilkes 
Exploring Expedition, the sloop of war Peacock sailed along 
the west coast of both Viti Lebu and Vanua Lebu, within the 
inner reefs, a distance exceeding two hundred miles. 

The island of Tahiti, on its northern side, presents a good 
illustration of a narrow channel, and at the same time one 
that exhibits the usual broken or interrupted character of 
reefs, This is seen in the following cut, in which the reefs, 
both fringing and barrier, are the parts enclosed by dotted 


STRUCTURE OF CORAL REEFS. 149 


Jines. The outer reef extends half to two-thirds of a mile 
from the shore. Within it, between Papieti and Matavai, 
there is an irregular ship channel, varying from three to 














CORAL REEFS OFF THE NORTH SHORE OF TAHITI, 


twenty fathoms in depth. Occasionally it enlarges into har- 
bors; and in other parts it is very intricate, though throughout 
navigable by large vessels. The island of Upolu, of the Sa- 
moan Group, is bordered by a reef nearly a mile wide on part 
of its northern shore; but the waters within are too shallow 
for a canoe at low tide; and therefore, notwithstanding its ex- 
tent, the reef is rather a fringing than a barrier reef. Within 
the green belt that encircles Bolabola (p. 138) there is a large 
and deep channel navigable by ships. 

Beneath these channels lies, in general, the coral rock of the 
reef-region—the inferior part of the great reef formation whose 
upper portions constitute the so-called barrier and fringing 
reefs. The rock would necessarily resemble that of the inner 
reefs already described; but there should be a larger propor- 
tion of the white compact limestone made from the fine coral 
sands carried off from the higher reefs by the currents. 


150 CORALS AND CORAL ISLANDS. 


Yet the bottoms of these channels are not always made up 
of calcareous or coral sands and fragments; for the volcanic 
or basaltic lands they adjoin are a source of ordinary mud; 
and the river courses of the land and the tidal currents of the 
sea will often determine the nature of the bottom, or may | 
cause in it alternate variations. 

At Upolu the white coral sands of the reefs (or in more 
general terms the reef débris), forms the bottom. In some 
places this coral material had the consistence of mud, and it 
was seldom observed to be covered with coarse material ; 
there were some small patches of coral over it, and here and 
there a growing mass of Porites. The fresh waters of the 
shores do not flow over these wide reefs, as there is no proper 
inner channel, and there is consequently no shore detritus 
mingled with the reef débris. 

At Tahiti, the sounding lead, where dropped in the channels, 
usually brought up sand, shells, and fragments of coral. At 
Tongatabu, the bottom where the Peacock anchored was a 
grayish blue calcareous mud, appearing as plastic as common 
clay ; it consisted solely of comminuted corals and shells, with 
coloring matter probably from vegetable and animal decom- 
position. 

But to the west of the larger Feejee islands, in the channels 
near the land, soundings commonly indicated a bottom of mud 
made from the material of the rocks of the mountains, and the 
same was frequently brought up with our dredges. On the 
north side of Vanua Lebu, a stream had so filled with its de- 
tritus the wide channel into which it empties, that for a mile 
the depth is but two to three fathoms, although elsewhere the 
depth is mostly from twelve to twenty fathoms; and at least 
half a dozen square miles of land had been added to the shores 
trom this source. Though due principally to shore material. 


STRUCTURE OF CORAL REEFS. 151 


the reefs have probably added somewhat to these accumula- 
tions; yet little coral sand could be detected in the mud by 
the eye, and the proportion is certainly very small. In many 
places where the ships of the Wilkes Exploring Expedition an- 
chored, having the reef not more than five hundred yards from 
the ship, the material of the bottom was wholly mud from 
the land, as much so as if there were no corals or shells with- 
in many miles. 

When the materials from both sources, the shore and the 
reef, are mingled, the proportion will necessarily depend on 
the proximity to the mouths of streams, the breadth of the 
inner waters or channels, and the direction and force of the 
currents. These tidal currents often have great strength, and 
are much modified and increased in force at certain places, or 
diminished in others, by the position of the reef with reference 
to the land. Sweeping on, they carry off the coral débris 
from some regions to others distant; and again they bear along 
and distribute only the shore detritus. It is thus seen that 
the same region may differ widely in its adjacent parts, and 
seemingly afford evidence in one place that there is no coral 
near, and in another no high land, although either is within a 
few rods, or even close alongside. 

The extent of the land in proportion to the reef will have 
an obvious effect upon the character of the channel or lagoon 
depositions. When the island stands, like one of Bacon’s isles 
in the Feejees, as a mere point of rock in a wide sea en- 
closed by a distant barrier, the streams of the land are small 
and their detritus quite limited in amount. In such a case, 
the reef, and the growing patches scattered over the lagoon, are 
the sources of nearly all the material that is accumulated upon 
the bottom. 


The bottom between the inner reefs within the great Aus- 


1 Ly CORALS AND CORAL ISLANDS. 


tralian barrier, according to Jukes, as brought up by the 
dredge from depths of fifteen to twenty fathoms, often resem- 
bles the unconsolidated mass of a shelly or coralline limestone. 
At other times it consisted very largely of the small disk-shaped 
foraminifers called Orbitolites, closely allied in form and na- 
ture to the Nummulites of the Tertiary; and they seemed 
in some places to make up the whole sand of the beaches, both 
of the coral islets and of the neighboring Australian shores. 

The facts show that the rock formed in such channels may 
be of all the kinds that occur in reef regions—coral and shell 
conglomerates, compact impalpable limestones, limestones full 
of Orbitolites, or containing, as well, remains of other species of 
the seas, and also rocks made of the clay, mud, sand or pebbles 
of the mountains or high lands adjoining. 


VI. BEACH SAND-ROCK. 


Besides the ordinary coral rock, there are also beach for- 
mations made of coral sands, worn shells, etc., thrown up by 
the tides and waves. ‘Their mode of formation is like that 
of any sea-beach. The material is mostly like common sand in 
fineness, but often much coarser. When the beach is fronted 
by a distant barrier to shield it from the force of the waves, 
the material is usually sand and small pebbles; but if the reef 
is narrow, so that the sea breaks over it with full force, it may 
consist even of cobble stones, as on any other shore, and in- 
clude also huge masses of coral rock. 

These deposits become cemented by being alternately mois- 
tened and dried, through the action of the recurring tides and 
the wash of the sea on the shores. The waters take up some 
carbonate of lime, and this is deposited and hardens among the 
particles on the evaporation of the moisture at the retreat of 
the tides. In some places the grains are loosely coherent, and 


STRUCTURE OF CORAL REEFS. 53 


seem to be united only by the few points in contact; and with 
a little care the calcareous coating which caused the union 
may be distinctly traced out. In other cases, the sand has 
been consolidated into a solid limestone rock, the interstices 
having been filled till a compact mass was formed. Generally 
even the most solid varieties show evidence of a sand origin, and 
in this they differ from the reef rock. The pebbly beds pro- 
duce a pudding stone of coral. 

In most localities the rock is an odlite or odlitic limestone. 
The grains become coated by the agglutinating carbonate of 
lime, and each enlarges thus into a minute sphere—a spherical 
concretion ; and the aggregation of these concretions makes 
the odlite. The grains are usually much smaller than the roe 
of most fishes, a resemblance which is alluded to in the name, 
from the Greek wor, egg. 

These beach deposits consist of regular layers, commonly 
from a few inches to a foot in thickness, and are generally con- 
solidated up to a line a little above high-tide mark. In all in- 
stances observed, the layers dip at an angle of six to eight de- 
grees down the beach. ‘This dip is nothing but the slope of the 
beach itself, and arises from the circumstance that the sands 
are deposited by the incoming waves, or tides, on such a slop- 
ing surface. Tutuila and Upolu, in the Navigator Group, and 
Oahu in the Hawaiian, afford many examples of these beach 
formations. At certain localities the beach sand-rock has been 
washed away after it was formed ; and occasionally large mass- 
es or slabs have been uplifted by the sea and thrown high up 
on the beach. 

Deposits of the same kind sometimes include detritus from 
the hills. Black basaltic pebbles are thus cemented by the 
white calcareous material, producing a rock of very singular 
appearance. Near Diamond Hill, on Oahu, is a good locality 


154 CORALS AND CORAL ISLANDS. 


for observing the steps in its formation. Many of the pebbles 
ot the beach are covered with a thin incrustation of carbonate 
of lime, appearing as if they had been dipped in milk, and 
others are actually cemented, yet so weakly that the fingers 
easily break them apart. 

The lime in solution in waters washing over these coral 
shores is also at times deposited in the cavities or seams of the 
volcanic rocks; thus the cavities of a lava or basalt become 
filled with white calcareous kernels, and the cellular’ lava is 
changed into an amygdaloid. In large cavities, or caverns, it 
often forms stalactites or stalagmitic incrustations. Similar 
facts are stated by Mr. Darwin as observed on the shores of 
Ascension; and many interesting particulars are given respect- 
ing calcareous incrustations on coasts in his work on Volcanic 
Islands, some of which are cited beyond. They were observed 
by the writer upon Madeira, in St. Jago, one of the Cape 
Verds, as well as among the volcanic islands of the Pacific. 

Jukes speaks of the odlitic character of the beach sand-rock 
about islets connected with the Australian barrier, and states 
“that the fact that the rock was not consolidated under wa- 
ter was proved by nests of turtles’ eggs being found imbedded 
in it, these evidently having been deposited by the animal 
when the sand was above water and still loose and incoherent.” 


Vil. DRIFT SAND-ROCK. 


Still another kind of beach formation is going on in some 
regions through the agency of the winds in connection with 
the sea., It occurs only on the windward side of islands when 
the reefs are narrow, and proceeds from the drifting of the 
sand into hillocks or ridges by the winds. 


The drifts resemble ordinary sand-drifts, and are often 





STRUCTURE OF CORAL REEFS. Ley 


quite extensive. On Oahu, they occur at intervals around the 
eastern shores, from the northern cape to Diamond Point, 
which forms the south cape of the island,—the part exposed to 
the trades ; and they are in some places twenty to forty feet 
in height. They are most remarkable on the north cape, a 
prominent point exposed to the winds that blow occasionally 
from the westward, as well as to the regular trades. They 
also occur on Kauai, another of the Hawaian Islands. But 
at Upolu (Samoa), where the protecting reefs are broad, the 
author met with no instance worthy of mention. 

These sand-banks, through the agency of infiltrating wa- 
ters, fresh or salt, become cemented into a sand-rock, more or 
less friable, which is frequently odlite. The rock consists of 
thin layers or lamine, which are very distinct, and indicate, 
generally, every successive drift of sand which puffs of wind 
had added in the course of its formation: and where a heavier 
gale had blown off the top of a drift, and new accumulations 
again completed it, the whole history is distinctly displayed. 

On northern Oahu, the elevated bluffs of coral-made 
limestone near Kahuku Point, eighty feet or so high 
above the sea-level, have a top of drift-sand rock, charac- 
teristic in its structure, resting on a much thicker coral- 
reef rock made up in part of large corals, some of them 
in the position of growth. Views of the bluffs showing 
the division between the two formations are given in the 
author's notes on Oahu, in his work on “ Volcanoes and 
the Hawaiian Islands.” 

The thickness and extent of drift-sand deposits depends 
on the character of the wind, the agent that makes them. 
Winds from one direction add a little to the height of a 
beach; from two or more, by sweeping off sands for con- 
tributions from a wider range of surface, make the height 


156 CORALS AND CORAL ISLANDS. aan 


: | eI 
often forty feet or more; and in a region subject to Vio- 4 | 
lent tempests or cyclones, like those of the Bahamas and/ | 
the Bermudas, they raise hills to a height of one or nce 
hundred feet, that are sometimes one or two miles broad, | | 

ae 
| 
| 
| 


and may make banks that are many miles in width. J 

About cavities over the surface, the rock is usually very 
compact to a depth of half an inch or more, almost horny 
in texture, owing to the infiltration of lime from the 
waters often occupying them; but this is an exceptional 
variety of the rock. 

Odlitic beds appear to be confined to the superficial forma- 
tions of a reef, that is, to the beach and wind-drift accumulations. \" 
No example has come under the notice of the author of odlite \ | 
constituting the foundation rock of a reef orisland. Itis possi- . 
ble that such beds might in some cases be the basement rocks to a 
considerable depth below ; for a reef-island might subside so | 
much more slowly than coral formations grow and accumulate, ( 
that a succession of beach-made beds would be produced even 
to a great thickness. Yet the probability is that the subsi- 
dence would sink the surface beneath the water, and put an 
end to beach and wind-drift work. The beach slope of 6° to 
8°. is an almost constant mark of beach-made beds. 


Vill. THICKNESS OF REEFS. 


We have considered in the preceding pages the peculiari- 
ties of form and structure characterizing the reef formations | 
bordering islands and continents, and their influence upon the 
enclosed land. Could we raise one of these coral-bound islands 
from the waves, we should find that the reefs stand upon the 
submarine slopes, like massy structures of artificial masonry ; 


some forming a broad flat platform or shelf ranging around 


STRUCTURE OF CORAL ISLANDS. 157 


the land, and others encircling it like vast ramparts, perhaps a 
hundred miles or more in circuit. The reefs that were near 
the water-line of the coast would be seen to have stood in the 
shallowest water, while the outer ramparts rested on the more 
deeply submerged slopes. Indeed, it is obvious that with a 
given slope to the declivity of the land, the thickness of the 
reef resting upon it nay be directly determined, as it would be 
twice as great two hundred feet from the shore as at one hun- 
dred feet. The only difficulty, therefore, in correctly determin- 
ing the depth or thickness of any given reef, arises from the 
uncertainty with regard to the submarine slope of the land. 
It is, however, admitted as the result of extensive observation, 
that in general these slopes correspond nearly with those of the 
land above water. Mr. Darwin has thus estimated the thick- 
ness of the reefs of the Gambier Group (p. 265) and some other 
Pacific islands, and he arrives at the conclusion, as his figures 
indicate, that some coral reefs, at their outer limits, are at least 
two thousand feet in thickness. 

The mountain slopes of the islands of the Pacific, except 
when increased by degrading agents, do not exceed in angle 
twelve or fourteen degrees, and they are often but half this 
amount. The slopes of Mauna Kea and Mauna Loa, isl- 
and of Hawaii, do not average over eight degrees. On the 
north side of Upolu, where the reefs are wide, the inclination 


is from three to six degrees. | Throughout the Pacific, the ~ 


steeper slopes of the mountains are due to agencies which can- 
not be shown to have affected the submarine slopes, excepting 
in cases of disruption of islands by forces below. 

Assuming eight degrees as the mean inclination, we should 
have for the depth of reef (or water), one mile from the shore, 
740 feet; or assuming five degrees, 460 feet. Adopting the 
first estimate, the Gambier Group would give for the outer 


_- 


158 CORALS AND CORAL ISLANDS. 


reef a thickness of at least 1,750 feet; or with the second, 1,150 
feet. The island of Tahiti (taking the north side for data) 
would give in the same manner 250 feet by the last estimate, 
which we judge to be most correct; Upolu, by the same esti- 
mate, 440 feet. The deduction for Upolu, may be too large: 
taking three degrees as the inclination, it gives 260 for the 
thickness at the outer margin. The results are sufficiently ac- 
curate to satisfy us of the great thickness of many barrier 
reefs. j 

These calculations, however, are liable to error from many 
sources. Very different results might generally be obtained 
from different sides of the same island; and the same group 
often contains islands without reefs, and others with reefs one 
or even several miles from the shores. But since we may show 
that the absence of a reef, or its limited extent, may be traced 
to some causes restricting or modifying its formation, it is ob- 
vious that the error would probably be on the side of too low 
an estimate. 

Adjacent to the larger islands, such as those of Vanua 
Levu, and Australia, the error might be of the opposite kind ; 
for the slopes of the land are of a more complex or irregular ~ 
character than on the smaller islands. In the latter, they may 
be shown to belong generally to a single elevation of igneous 
origin, or,at the most, to two or three combined; while, in the 
former, they may pertain to different ranges of hills or moun- 
tains. | For correct results in any instance, the land and its 
declivities should be carefully studied beforehand, and the sys- 
tem in its inclinations determined by observation. With re- 
gard to Tahiti and Upolu, information bearing upon this point 
was obtained, and the above conclusions may be received with 
much confidence. Many of the Feejee reefs, on the same prin- 
ciple, cannot be less than 2,000 feet in thickness. 


STRUCTURE OF CORAL REEFS. 159 


Ix. A GOOD WORD FOR CORAL REEFS. 


All coral-bound coasts, and especially those of islands in mid 
ocean, derive great benefit from their reefs. The wide coral banks 
and the enclosed channels greatly enlarge the limits tributary 
to the lands they encircle. Besides being barriers against the 
ocean, they are dykes to detain the detritus of the hills. 
They stop the waters of the streams, and cause it to drop the 
silt they were bearing off, and thus secure its addition to the land. 
They prevent, therefore, the waste which is constantly going on 
about islands without such barriers ; for the ocean not only en- 
croaches upon the unprotected shores of small islands, but car- 
ries off much of whatever the streams empty into it. The del- 
ta of Rewa, on Viti Lebu, resulting from the detritus accumu- 
lations of a large river, covers nearly sixty square miles. This 
is an extreme case in the Pacific, as few islands are so large, 
and consequently rivers of such magnitude are not common. 
But there is rarely a coral-girt island which has not at least 
some narrow plains from this source; and upon them the vil- 
lages of the natives are usually situated. Around Tahiti these 
plains are from half a mile to two or three miles in width, and 
the cocoa-nut and bread fruit groves are mostly confined to 
them. 

The reefs also provide extensive fishing-grounds for the na- 
tives, and afford abundant fish, their main reliance in the way 
of animal food. They also supply large interior waters for 
practice in navigation and for safe communication between dis- 
tant settlements. And the effect is evident in the spirit of 
maritime enterprise which characterizes the islanders; for 
these circumstances have favored the construction of large sail- 
canoes in which they venture beyond their own land, and often 
undertake voyages hundreds of miles in length. Communica- 


160 CORALS AND CORAL ISLANDS. 


tion between the Friendly Islanders and the Feejees has long 
been kept up by means of these large rudely-rigged sail-canoes. 

Instead of a rock-bound coast, harborless and thinly hab- 
itable, like St. Helena, in the tropics, and nearly all extra- 
tropical islands, the shores of these reef-bound lands are bloom- 
ing to the very edge, and wide plains are spread out with 
bread fruit and other tropical productions. Harbors, safe for 
scores of vessels, are also opened by the same means; and 
some islands number a dozen, when the unprotected shores 
would hardly have afforded a single good anchorage. Jukes 
remarks that the sea within the great Australian barrier is 
‘one great natural harbor;” and this harbor is as long as 
from the extremity of Florida to Newfoundland. 

Coral-reefs are sometimes viewed as only traps to sur- 
prise and wreck the unwary mariner; but whoever has vis- 
ited the dreary prison-house, St. Helena, will have some appre- 
ciation of the benefits derived from the growing zoéphytes. 

But in addition to these general, benefits, there are also 
contributions from the larger reef regions to the commerce 
of the world. Besides pearls, there is the biche de mar 
(called also, béche de mer, sea-ginseng, and in China, tripang), 
thousands of hundred-weight of which annually enter the 
Chinese market from the reef-regions of the Kast Indies, Aus- 
tralia, and the seas to the north, including the Feejee Archi- 
pelago. This favorite material for Chinese dishes, either stews 
or soups, etc, is dred holothuria—large slug-like animals, 
called often sea slugs, and also sea cucumbers, from their form 
in the contracted state. They are not slugs, but are most 
nearly related to the echinus, though having a thick flexible 
skin, while the echinus has for its exterior a firm shell, armed 
about with spines. The largest are only nine or ten inches 
long when contracted ; but they lengthen out sometimes to 


STRUCTURE OF CORAL ISLANDS. 16] 


two feet or more. They live just under the sand in the shal- 
low waters, with the head projecting and bearing a beautiful 
feathery rosette or flower which is branchial in nature. To 
fit them for exportation, the holothuria, of which half a dozen 
different kinds are taken, are slit open, boiled, and then dried, 
in which last state they look like “‘smoked sausages. Dr. S. 
Wells Williams says, in his ‘‘ Middle Kingdom,” that ‘ when 
soaked in water, the material resembles pork rind, and is like 
that in taste when stewed.” They are brought to China by the 
Malays from Macassar, and elsewhere. ‘There are also large 
drying-houses at the Feejees, and ships from America make 
their occasional visits to collect them, with the aid of the Fee- 
jees, and to dry and load up for China. The term biche de mar, 
and also the French form of it, béche de mer, are corruptions of 
the Portuguese bicho do mar, which means sea-worm or sea-slug. 


Il. STRUCTURE OF CORAL ISLANDS. 


I. FORMS AND GENERAL FEATURES. 


Coral islands resemble the reefs just described, except that 
a lake or lagoon is encircled instead of a mountainous island, _ 
A narrow rim of coral reef, generally but a few hundred yards 
wide, stretches around the enclosed waters. In some parts 
the reef is so low that the waves are still dashing over it into 
the lagoon ; in others it is verdant with the rich foliage of the 
tropics. ‘The coral-made land, when highest, is seldom more 
than ten or twelve feet above high tide. 

When first seen from the deck of a vessel, only a series of 
dark points is descried just above the horizon. Shortly after 
the points enlarge into the plumed tops of cocoa-nut trees, and 
a line of green, interrupted at intervals, is traced along the 


water's surface. Approaching still nearer, the lake and its belt 
1] 


162 CORALS AND CORAL ISLANDS. 


of verdure are spread out before the eye, and a scene of more 
interest can scarcely be imagined. ‘The surf, beating loud and 
heavy along the margin of the reef, presents a strange contrast 
to the prospect beyond,—the white coral beach, the massy 
foliage of the grove, and the embosomed lake with its tiny 
islets. The color of the lagoon water is often as blue as the 
ocean, although but ten or twenty fathoms deep; yet shades 
of green and yellow are intermingled, where patches of sand 
or coral-knolls are near the surface ; and the green is a delicate 
apple-shade, quite unlike the ordinary muddy tint of shallow 


waters. 
































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































CORAL ISLAND, OR ATOLL. 


The belt of verdure, though sometimes continuous around 


the lagoon, is usually broken into islets separated by varying 


intervals of bare reef; and through one or more of these in- 
tervals, a ship-channel often exists opening into the lagoon. 
The larger coral islands are thus a string of islets along a 
line of reef. | 

These lagoon islands are called atolls, a word of Maldive 
origin. The king of the Maldives bears the high sounding 
title of ‘“‘ Ibrahim Sultan, King of the thirteen Atollons and 
twelve thousand Isles (see page 189); which Capt. W. F. W. 
Owen, R. N., says is no exaggeration. 

In the larger atolls, the waters within look like the ocean, 
and are similarly roughened by the wind, though not to the 








STRUCTURH OF CORAL ISLANDS. 163 


| 


same extent.. Standing on the north shore of the Raraka la- 
goon and looking southwest, nothing is seen but blue waters. 
Far in the distance to the right, and also to the left, a few 
faint dots are observed; and as the eye sweeps around in 
either direction, these dots gradually enlarge and pass into lines 
of verdure, and finally, distinct groves near the observer. At 
Dean’s Island, another of the Paumotus, and at some of the 
Carolines, the resemblance to the ocean is still more striking. 
The lagoon is in fact but a fragment of the ocean cut off by 
more or less perfect walls of coral reef-rock; and the reef is 
here and there surmounted by verdure, forming a series of 
islets. 

In many of the smaller coral islands, the lagoon has lost 
its ocean character, and become a shallow lake, and the green 
islets of the margin have coalesced in some instances into a 
continuous line of foliage. Traces may perhaps be still de- 
tected of the passage, or passages, over which the sea once com- 
municated with the internal waters, though mostly concealed 
by the trees and shrubbery which have spread around and 
completed the belt of verdure. The coral island is now in its 
most finished state; the lake rests quietly within its circle of 
palms, hardly ruffled by the storms that madden the sur- __ 
rounding ocean. ibs 

From the islands with small lagoons, there is every variety 
in gradation down to those in which there is no trace of a la- 
goon. These simple banks of coral are the smallest of coral 
islands. In all the larger islands the windward side is the 
highest ; and sometimes it is wooded and habitable through- 
out when the leeward reef is bare. The entrances to the la- 
goons are accordingly on the leeward side. 

A single group of islands, the Gilbert or Kingsmill, af- 
fords good examples of the principal varieties. It is at once 


— 


164 CORALS AND CORAL ISLANDS. 


) 


seen from these examples that atolls are not annular, In the 
southernmost, Tapateuea, the form 1s very narrow, the 
length being thirty-three miles, with the width of the southern 
portion scarcely exceeding six miles, and that of the northern 
more than one-half less. The emerged land is confined to one 
side, the eastern or windward, and consists of a series of islets 
upon the eastern line of coral reef. The western side is for 
the most part several feet under water, and there is hardly a 
proper lagoon. Sailing by the island, to windward, the patch- 
es of verdure, thus strung together, seem to rise out of a long 
white line of breakers, the sea surging violently against the un- 
seen coral reef upon which they rest. 

Nonouti, the next island north, is about twenty miles 
long by eight broad. The rim of Jand, though in fewer islets, 
is similar to that of Tapateuea in being confined to the reef 
fronting northeast. The reef of the opposite side, though bare 
of vegetation, stands near low-tide level, and the whole en- 
closes a large lagoon. 

Aranuka and Apamama, though smaller than Nonouti, 
have the same general character. Aranuka is triangular in 
shape, and has an islet on the western point or cape, which is 
quite prominent. Apamama differs from either of the preced- 
ing in having two narrow ship entrances to the lagoon, one 
through thé northwestern reef, and another through the south- 
western. 

Kuria is a remarkable double island, without a proper 
lagoon. It consists of two neighboring groves, each about a 
square mile in extent, on adjacent patches of reef. 

Maiana is quite regularly quadrangular, with an uninter- 
rupted range of land on two of the four sides, and an exposed 
reef constituting the other two. 

Tarawa consists of two sides of a triangle. The western 





Plate VIII. 








x 
~ Marana 





Nonouti 


, 
e . 








GILBERT OR KINGSMII.IL ISLANDS 














STRUCTURE OF CORAL ISLANDS. 167 


reef is wanting, and the sea and lagoon have unbroken com 
munication. In place of it, there are two to ten fathoms of 
water, and a bottom of coral sand. Small vessels may sail in 
almost anywhere on this side to good anchorage, and there is 
a passage for ships of the largest size. The depth within is 
greater than on the bar, and these inner waters obviously cor- 
respond to the lagoon of other islands. 

Apaiang has much resemblance to Apamama in its forest 
border and lagoon. Moreover, there is a ship entrance through 
the southwestern reef. 

Marakei is one of the prettiest coral islands of the Pacific. 
The line of vegetation is unbroken. In a view from the mast- 
head it lies like a garland thrown upon the waters; the un- 
practiced eye scarcely perceives the variation from a circular 
form, however great it may be. The grove is partially inter- 
rupted at one point, where there are indications of a former 
passage through the reef. 

Tari-tari, lying to the north of Apia, is a large triangular 
atoll. It is wooded almost continuously on the side facing 


southeast, and has a few spots of verdure on the southwest, . 


with three entrances to the extensive lagoon. The northern 
side is a naked reef throughout, scarcely apparent from a ship’s 
deck, except by the long line of breakers. Makin, just north 
of Tari-tari, is a mere patch of coral reef without a lagoon. 

Tapateuea is also called Drummond’s Island ; Nonouti 
is Sydenham; Aranuka is fenderville, and Apamama is 
Hopper Island. 

We add a few more descriptions of Pacific islands, with 
figures reduced from the maps of the Wilkes Expedition to a 
scale of four-tenths of an inch to a mile. 

Taiara and Henuake (figs. | and 2) are two small belts 
of foliage, somewhat similar to Maraki. 


dl 


7a 


168 CORALS AND CORAL ISLANDS. 


Swain’s and Jarvis’s Islands (figs. 3 and 4), are of still 
smaller size, and have no lagoon. ‘The former is densely 
covered with foliage, while the surface of the latter is sandy. 
Both islands are a little depressed about the centre, a fact 
indicating that there was formerly a lagoon. 


1 





TAIARA, HENUAKE, OR HONDEN, 


Fakaafo, or Bowditch (fig. 5), 200 miles north of the 
Navigator Islands, is the type of a large part of coral islands. 
The bank of reef has only here and there emerged from the 








JARVIS’S ISLAND. 





SWAIN’S ISLAND. 








FAKAAFO, OR BOWDITCH’S ISLAND. | 


waves and become verdant; in other portions the reef is of 
the usual height,—that is, near low-tide level,—excepting a 
few spots elevated a little by the accumulation of sand, 





STRUCTURE OF CORAL ISLANDS. 169 


The Paumotu. Archipelago, the crowded cluster of coral 
islands east and northeast of ‘Tahiti, is a most instructive study 
for the reader; and a map of these islands by the Wilkes Ex- 
ploring Expedition, inserted in the Narrative of the Expedi- 
tion, and also in the Hydrographical Atlas, will well repay 
close examination. Sailing among these islands, over eighty 
in number,—only four of which are over twelve feet high exclu- 
sive of the vegetation,—two or three are almost constantly in 
sight from the mast-head. 

The small amount of habitable land on these reef-islands 
is one of their most peculiar features. Nearly the whole sur- 
face is water; and the land around the lagoon is but a narrow 
rim, the greater part of which is usually under water at high 
tide. This fact will be rendered more apparent from the fol- 
lowing table, containing a statement of the sizes and areas 
of several islands, with the amount of habitable land. The 
measures are given in geographical miles. 


GREATEST AREA IN “ABITABLE 
LENGTH. GreaDra, 99, tus, PARTS IN 
Carlshoff, Paumotus, 25, 13 200 10 
Wolchonsky, ‘“ 15 3 40 3 
Raraka, oe 15 10 90 8 
Manhii, “ 14 64 50 9 
Nairsa or Deans, Paumotus, 50 Me) 1000 16 
Fakaafo, Union Group, + 43 20 24 
Duke of Clarence, “ 84 5s 21 2 
Tapateuea, Kingsmills, 33 6 60 6 
Tarawa, “ 20 10 130 8 
Nonouti, ie 22, 9 125 7 
Tari-tari, e 18 11 110 + 


The ten islands here enumerated have an ageregate area 
of 1,852 square miles, while the amount of actual dry habit- 
able land is but seventy-six miles, or less than one twenty- 
fourth. In the Caroline Archipelago the proportion of land 
is still smaller. Menchikoff atoll covers an area of 500 square 


170 CORALS AND CORAL ISLANDS. 


miles, and includes hardly six square miles of wooded land. 
In the Marshall Islands the dry land is not over one-hundredth 
of the whole surface; while in the Pescadores the proportion 
of land to the whole area is about as 1 to 200. 

The distribution of the land upon the reef is obvious from 
the sketches already given. It is seen, as long since remarked. 





MENCHICOFF ATOLL. 
1-20 of an inch to a mile. 


that the windward side is, in general, the highest. It is also 
apparent that there are not only great irregularities of form, 
but that on one side the reef may at times be wholly wanting 
or deeply submerged. 

In many islands there is a ship-entrance through the reef, 
sometimes six or eight fathoms deep, to the lagoons, where 
good anchorage may be ‘had; but the larger part have only 
shallow passages, or none at all. In the Paumotus, out of the 
twenty-eight visited by the Expedition, not one-half were 
found to have navigable entrances. In the Carolines, where 
the islands are large and not so much wooded, entrances are 
of more common occurrence. About half of the Kingsmill 
Islands afford a good entrance and ‘safe anchorage. Through 


these openings in the reefs, there is usually a rapid outward 


STRUCTURE OF CORAL ISLANDS. 171 


current, especially during the ebbing tide. At Depeyster 
Island, it was found to run at the rate of two and a half miles 
an hour. It was as rapid at Raraka, in the Paumotus, and, as 
Capt. Wilkes remarks, it was difficult to pull a boat against 
tt into the lagoon. 


Il. SOUNDINGS ABOUT CORAL ISLANDS. 


The water around coral islands deepens as rapidly and in 
much the same way as off the reefs about high islands. The 
atoll usually seems to stand as if stilted up in a fathomless sea. 
The soundings of the Expedition afford some interesting re- 
sults. 

Seven miles east of Clermont Tonnerre, the lead ran out to 
1,145 fathoms (6,870 feet), without reaching bottom. Within 
three quarters of a mile of the southern point of this island, the 
lead, at another throw, after running out for a while, brought 
up an instant at 350 fathoms, and then dropped off again and 
descended to 600 fathoms without reaching bottom. On the 
lead, which appeared bruised, a small piece of white coral was 
found, and another of red; but no evidence of living z00- 
phytes. On the east side of the island, three hundred feet 
from the reef, a bottom of coral sand was found in 90 fathoms; 
at one hundred and eighty feet, the same kind of bottom in 85 
fathoms. 

Off the southeast side of Ahii (another of the Paumotus), 
about a cable’s length from the shore, the lead, after descend- 
ing 150 fathoms, struck a ledge of rock, and then fell off and 
finally brought up at a depth of 300 fathoms. 

Two miles east of Serle’s Island, no bottom was found at 
600 fathoms. 

A mile and a half south of the larger Disappointment 
Island, there was no bottom at 550 fathoms. 


172 CORALS AND CORAL ISLANDS. 


Near the eastern end of Metia, an island nearly north of 
Tahiti, no bottom was found with a line of 150 fathoms; and, 
a mile distant, no bottom was reached at 600 fathoms. 

In general, for one to five hundred yards from the 
margin of the shore reef, the water slowly deepens, and 
then there is an abrupt descent at an angle of 40 or 50 
degrees. The results of earlier voyagers correspond with 
this statement. 

Beechey, whose observations on soundings are the fullest 
hitherto published, states many facts of great interest. At 
Carysfort Island, he found the depth, 60 yards from the surf 
line, 5 fathoms; 80 yards, 15 fathoms; 120 yards, 18 fath- 
oms; 200 yards, 24 fathoms; and immediately beyond, no 
bottom with 35 fathoms. At Henderson’s Island, soundings 
continued out 250 yards, where the depth was 25 fathoms, 
and then terminated abruptly. Off Whitsunday, 500° feet 
out, there was no bottom at 1,500 feet. 

Darwin states other facts bearing upon this subject, of 
which we may cite the following: At Heawandoo Pholo (one 
of the Maldives), Lieutenant Powell found 50 or 60 fathoms 
close to the edge of the reef. One hundred fathoms from the 
mouth of the lagoon of Diego Garcia, Captain Moresby found 
no bottom with 150 fathoms. At Egmont Island, 50 fath- 
oms from the reef, soundings were struck in 150 fathoms, At 
Cardoo Atoll, only 60 yards from the reef, no bottom was ob- 
tained with a line of 200 fathoms. Off Keeling Island, 2,200 
yards from the breakers, Captain Fitzroy found no bottom 
at 1,200 fathoms. Mr. Darwin also states that, at a depth 
between five and six hundred fathoms, the line was partly 
cut as if it had rubbed against a projecting ledge of rock; 
and deduces from the fact “the probable existence of sub- 


marine cliffs.” 























CENTRAL PACIFICO 


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2000 fath.... 3000 fath 























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STRUCTURE OF CORAL ISLANDS. Wes 


In the Phoenix Group (Plate IX.), depths of 3,000 to 

3,300 fathoms occur, and 3,000 half way between Sydney’s and 
Birnie’s, sixty miles apart; to the southwest of Hnderbury’s 

slopes of 1 : 6 and 1: 3 exist, and to the northeast, of 1: 1:5 
and 1:4. Off Swain’s Island, slopes of 1:7 and 1:13 
were obtained; and off Danger Island (same Plate), 660 
fathoms within half a mile southwest of the reef, and 985 
fathoms one mile east, giving slopes of | : 1 and 1 : 0-75. 

Off the Bahamas, for 400 miles, depths of 2,500 to 3,000 
fathoms occur within twenty miles of the reefs, and at one 
point 2,336 fathoms within two miles, a pitch down of 
1: 0°75. South of central Cuba a depth of 3,428 fathoms exists 
within twenty miles of the Grand Cayman reef, and 3,010 
fathoms within fifteen miles of Swan Island reef. 

There are examples also of less abrupt slopes. Northwest 
of the Hawaian Group, Captain Lisiansky, who commanded 
the Russian ship Neva in a voyage round the world in the 
years 1860-61, at the island bearing his name found shallow 
water for a distance of six or seven miles; the water deepened 
to ten or eleven fathoms the first mile, fifteen the second, and at 
the last throw of the lead there were still but twenty-five fath- 
oms. Christmas Island affords on its western side another 
example of gradually deepening waters. Yet these shallow 
waters terminate finally in a rapid declivity of forty or 
fifty degrees. 

Off the prominent angles of an atoll, soundings gener- 
ally continue much beyond the distance elsewhere, as was 
first observed by Beechey. At Washington Island, mostly 
abrupt in its shores, there is a bank, according to the sur- 
veys of the Expedition, extending from the east point to 
a distance of half a mile, and another on the west extend- 


ing toa distance of nearly two miles. At Kuria, one of 


et CORALS AND CORAL ISLANDS 


the Kingsmills, soundings continue for three miles from the 
north extremity, along a bank stretching off from this point 
to the north-northwest. 


Ill. STRUCTURE OF CORAL ISLANDS. 


The descriptions of reefs and their islets already given apply © 
with equal force to coral islands. By transferring here the 
statements respecting the former, we should have a nearly 
complete account of the latter. ‘The same causes, with scarcely 
an exception, are at work :—the growing of coral zodphytes, and 
the action of the waves, of oceanic currents, and of the winds. 
This resemblance will be rendered more apparent by a review 
of their characters. The description will be found to be a sim- 
ple recapitulation of a former paragraph. 

The reef of the coral atoll, as it lies at the surface still 
uncovered with vegetation, is a platform of coral rock, usually 
two to four hundred yards wide, and situated so low as to be 
swept by the waves at high tide. The outer edge, directly 
exposed to the surf, is generally broken into points and 
jagged indentations, along which the waters of the resurging 
wave drive with great force. Though in the midst of the 
breakers, the edge stands a few inches, and sometimes a foot, 
above other parts of the platform; the incrusting Nullipores 
cover it with varied tints, and afford protection from the 
abrading action of the waves, There are usually three to five 
fathoms water near the “margin ; and below, over the bottom, 
which gradually degnans outward, beds of corals are growing 
profusely among extensive patches of coral sand and frag- 
ments. Generally the barren areas much exceed those flour- 
ishing with zoéphytes, and not unfrequently the clusters are 
scattered like tufts of vegetation in a sandy plain. The grow- 
ing corals extend up the sloping edge of the reef, nearly to 


STRUCTURE OF CORAL ISLANDS. DS 


low-tide level. For ten to twenty yards from the margin, the 
reef is usually very cavernous or pierced with holes or sinuous 
recesses, a hiding-place for crabs and shrimps, or a retreat for 
the echini, asterias, sea-anemones and mollusks ; and over this 
portion of the platform, the gigantic Tridacna, sometimes over 
two feet long, and 500 pounds in weight, is often found lying 
more than half buried in the solid rock, with barely room to 
gape a little its ponderous shell, and expose to the waters a 
gorgeously colored mantle. Further in are occasional pools 
and basins, alive with all that lives in these strange coral seas. 

The reef-rock, when broken, shows commonly its detritus 

origin. Parts are of compact homogeneous texture, a solid 
white limestone, without a piece of coral distinguishable, and 
rarely an imbedded shell. - But generally the rock is a breccia 
or conglomerate, made up of corals cemented into a compact 
mass, and the fragments of which it consists are sometimes 
many cubic feet in size. 

It is apparent that we are describing a second time an 
outer reef. Without dwelling further upon its characters, we 
may pass to the features of the reef when raised above the 
waters and covered with vegetation. 

Sections of coral islands and their lagoons have been given 
by Captain Beechey and Mr. Darwin. We add another, by 
way of illustration, although little may be presented that 1s 
novel after the excellent descriptions of these authors. Sketch- 
es of several of these islands, showing the general relation of 
the rim of land to the reef and the lagoon within, are given 
in the maps of islands on pages 165, 168. The following sketch 
represents a section of the rim of land from the sea on one 
side (the left), to the lagoon on the other. In the view, the 
part m a represents the shallow sea bordering an island, and 
abruptly deepening one to six hundred feet from the line of 


{76 CORALS AND CORAL ISLANDS. 


breakers. In these shallow waters are the growing corals; 
yet, as before stated, a large part is often barren sand or coral 
rock, especially where the depth is over fifty feet. 








WMléuélllltétt-le 





ti 
VLE 


is g SECTION’OF “THE RIM OF AN ATOLL. 


MMMM 
d € n 


From « to 6 is the shore platform or reef-rock, nearly at 
low-tide level, with the margin (@) slightly elevated, and usually 
much incrusted at top with Nullipores. From the platform 
there is a rise, by a steep beach () c), of six or eight feet, to 
the wooded part of the coral belt represented between ¢ and d. 
From d to ¢ there is a gently sloping beach bordering the 
lagoon. Beyond e, the waters of the lagoon at first deepen 
gradually, and then fall off more or less abruptly. 

In the Paumotus, the shore platform, the steep beach, and 
the more gently sloping shore of the lagoon are almost con- 
stant characteristics. 

The width of the whole rim of land, when the island gives 
no evidence of late elevation, varies from three hundred yards 
to one-third of a mile, excepting certain prominent points, more 
exposed to the united action of winds and waves and often 
from opposite directions, which occasionally exceed half a mile. 

The shore platform is from one to three hundred feet in 
width, and has the general features of a half-submerged outer 
reef. Its peculiarities arise solely from the accumulations 
which have changed the reef into an island. Much of it is 
commonly bare at low tide, although there are places’where it 
is always covered with a few inches or a foot of water; and 
the elevated edge, the only part exposed, often seems like an 


embankment. preventing the water from running off. The 


STRUCTURE OF CORAL ISLANDS. LTP 


tides, as they rise, cover it with water throughout, and bear 
over it coral fragments and sand, comminuted shells and other 
animal remains, to add them to the beach. ‘The heavier seas 
transport larger fragments; and at the foot of the beach there 
is often a deposit of blocks of coral, or coral rock, a cubic foot 


or so in size, which low tide commonly leaves standing in a _ 


few inches of water. / On moving these masses, which gener- 
ally rest on their projecting angles and have an open space 
beneath, the waters at once become alive with fish, shrimps, 
and crabs, escaping from their disturbed shelter; and beneath, 
appear various Actiniz or living flowers, the spiny echini and 
sluggish biche-de-mar, while swarms of shells, having a soldier 
crab for their tenant, walk off with unusual life and stateliness. 
Moreover, delicate corallines, Ascidiz and sponges tint with 
lively shades of red, green and pink, the under surface of the 
block of coral which had formed the roof of the little grotto. 
Besides the deep channels. cutting into the margin of the 
reef and giving it a broken outline, there are in some instances 


long fissures intersecting its surface. On Aratica (Carlshoff), 
and Ahii (Peacock Island), they extended along for a fourth 
to half a mile, generally running nearly parallel with the 
shore, and at top were from a fourth to half an inch wide. 
These fissures are not essential features of the reef. They are 
probably a result of a subterranean movement or shaking. 


The beach consists of coral pebbles or sand, with some 


worn shells, and occasionally the exuvie of crabs and bones 
of fishes. Owing to its whiteness, and the contrast it affords 
to the massy verdure above, it is a remarkable feature in the 
distant view of these islands, and often seemed like an arti- 


ficial wall or embankment running parallel with the shores. . 


On Clermont Tonnerre, the first of these islands visited by us, 


the natives seen from shipboard, standing spear in hand along 
12 


—kK 7 


“ 


x 


178 CORALS AND CORAL ISLANDS. 


the top of the beach, were believed by some to be keeping 
patrol on the ramparts of a kind of fortification. This decep- 
tion arose from the dazzling whiteness of the coral sand, in 
consequence of which, the slope of the beach was not distin- 
guished in the distant view. 

The emerged land beyond the beach, in its earliest stage, 
when barely raised above the tides, appears like a vast field of 
ruins. Angular masses of coral rock, varying in dimensions 
from one to a hundred cubic feet, lie piled together in the 
utmost confusion; and they are so blackened by exposure, or 
from incrusting lichens, as to resemble the clinkers of Mauna 
Loa; moreover, they ring like metal under the hammer. / Such 
regions may be traversed by leaping from block to block, with 
the risk of falling into the many recesses among the huge 
masses. On breaking an edge from the black masses, the 
usual white color of coral is at once apparent. Some of the 
blocks, measuring five or six feet in each of their dimensions, 
were portions of single individual corals; while others had the 
usual conglomerate character of the reef-rock, or, in other 
words, were fragments torn by the waves from the reef-rock. 

In the next stage, coral sand has found lodgment among 
the blocks; and although so scantily supplied as hardly to 
be detected without close attention, some seeds have taken 
root, and vines, purslane, and a few shrubs have begun to 
grow, relieving the scene, by their green leaves, of much of 
its desolate aspect. : 

Both of these stages are illustrated on the greater part of 
coral islands. 

In the last stage, the island stands six to ten feet out of 
water. The surface consists of coral sand, more or less dis- 
colored by vegetable or animal decomposition. Scattered 
among the trees, stand, still uncovered, many of the larger 


STRUCTURE OF CORAL. ISLANDS. 179 


blocks of coral, with their usual rough angular features and 
blackened surface. ‘There is but little depth of coral soil, 
although the land may appear buried in the richest foliage. 
In fact, the soil is scarcely any thing but coral sand. It is 
seldom discolored beyond four or five inches, and but little of 
it to this extent; there is no proper vegetable mould, but only 
a mixture of darker particles with the white grains of coral 
sand. It is often rather a coral gravel, and below a foot or 
two, it is usually cemented together into a more or less com- . 
pact coral sand-rock. 

One singular feature of the shore platform, occasionally 
observed, remains to be mentioned. Huge masses of reef- 
rock are sometimes found upon it, some of which lie loose 
upon the reef, while others are firmly imbedded in it below, 
and so cemented to it as to appear to be actually a part of the 
platform rock. Sketches of two of these masses are here given. 
































BLOCKS OF OORAL ROCK ON THE SHORE PLATFORM. 


Figure 1 represents a mass on the island of Waterland 
(one of the Paumotus), six-feet high and about five in diam- 
eter; it was solid with the reef-rock below, as though a part 
of it, and, about two feet above its base, it had been so nearly 
worn off by the waters as to have become irregularly top- 
shaped. Another mass, similarly attached to the reef at base, 
observed on Kawehe (Vincennes Island), was six feet high 
above low-water level, and seven feet in its longest diameter 


180 CORALS AND CORAL ISLANDS. 


Below, it had been worn like the one just described, though 
to a less extent. Another similar mass was eight feet high. 
Figure 2 represents a block six feet high and ten feet in its 
longest diameter, seen on Waterland; it was unattached be- 
low, and lay with one end raised on a smaller block. On 
Aratica (Carlshoff), others were observed. One loose mass 
like the last was eight feet high and fifteen feet in diameter, 
and contained at least a thousand cubic feet. Raraka also 
afforded examples of these attached and unattached blocks, 
some standing with their tops six feet above high-water mark. 

These masses are similar in character to many met with 
among the fields of blocks just described, and differ only in 
having been left on the platform instead of transported over it. 
Some of them are near the margin of the reef, while others are 
quite at itsinner limit. The second mass alluded to above, on 
Kawehe, was a solid conglomerate, consisting of large fragments 
of Astreeas and Madrepores, and contained some imbedded 
shells, among which an Ostrea and a Cyprea were noticed. 
This is their usual character. The other two were parts of 
large individual corals (Porites); but there was evidence in the 
direction of the cells that they did not stand as they grew; on 
the contrary, they had been upthrown, and were afterward 
cemented with the material of the rock beneath them, probably 
at the time this rock itself was consolidated. Below some of 
the loose masses the platform was at times six inches higher 
than on either side of the mass, owing to the protection from 
wear given to the surface beneath it. These blocks are always 
extremely rough and uneven, like those of the emerging land 
beyond; and the angular features are partly owing, in both 
cases, to solution from rains and from the dashes of sea-water 
to which, with every tide, they are exposed. 

It should be distinctly understood that these masses here 


¥ x 
La 
i 


STRUCTURE OF CORAL ISLANDS. 181 


described were found isolated, and only at considerable inter- 
vals. In no instance were they observed clustered. The loose 
blocks and those cemented below had the same general charac- 
ter, and must have been placed where they were by the same 
cause, though it may have been at different periods. 

Such blocks are of course not confined to coral island reefs, 
but belong to barrier reefs generally. 

Jukes says, ‘I once landed close to the edge of the Aus- 
tralian barrier on the south side of the Blackwood channel, in 
south latitude 11° 45’, on a continuous mass of Porites which 
was at least twenty feet across, and it seemed to pass down- 
wards into the mass of the reef below water without any dis- 
connection. It was worn into pinnacles above, so that two or 
three of us could stand in the different hollows without seeing 
each other; and it was one of a line of such masses that at- 
tracted our attention for a distance of three miles.” 

The shore of the lagoon is generally low and gently in- 
clined, yet in the larger islands, in which the waters of the 
lagoon are much disturbed by the winds, there is usually a 
beach resembling that on the seaward side, though of less 
extent. A platform of reef-rock at the same elevation as 


the shore platform sometimes extends out into the lagoon; | 
but it is more common to find it a little submerged and coy- | 
ered for the most part with growing corals; and in either case, | 
the bank terminates outward in an abrupt descent, of a few. 
yards or fathoms, to a lower area of growing corals, or a bot- | 


tom of sand. Still more commonly, we meet with a sandy 
bottom gradually deepening from the shores without growing 
coral. These three varieties of condition are generally found 
in the same lagoon, characterizing its different parts. The 
lower area of growing corals slopes outward, and ceases where 
the depth is 10 to 12 fathoms or -sooner; from this there 


182 CORALS AND CORAL ISLANDS 


is another descent to the depth which prevails over the lagoon. 
On some small lagoons the shore is a thick plastic mud, 
either white or brownish, and forms a low flat which is very 
gently sloping. On Henuake, these mud deposits are quite ex- 
tensive, and of a white color. At Enderbury’s Island, another 
having a shallow lagoon, the mud was so deep and thick that 
there was some difficulty in reaching the waters of the lagoon ; 
the foot sunk in eight or ten inches and was not extricated 
without some difficulty. It looked like a dirty brownish clay. 
This mud is nothing but comminuted coral, so fine as to be 
almost impalpable. 

The lagoons of the smaller islands are usually very shallow ; 
and in some, merely a dry bed remains, indicating the former 
existence of water. Instances of the latter kind are met with 
only in islands less than three miles in diameter; and those 
with shallow lagoons are seldom much larger. ‘These shallow 
waters, when direct communication with the sea is cut off, be- 
come, in some instances, very salt by evaporation, and contain 
no growing coral, with few signs of life of any kind; and in 
other cases, they are made too fresh for marine life through 
the rains. At Enderbury’s Island the water was not only ex- 
tremely saline, but the shores of the lagoon were in some 
places incrusted with salt. But when there is an open channel, 
or the tides gain access over a bare reef, corals continue to 
grow, and a considerable portion of the lagoon may be obstruct- 
ed by them. At Henuake, the sea is shut out except at high 
water, and there were consequently but few species of corals, 
and those of small size. At Ahii (Peacock’s Island), there 
was a small entrance to the lagoon, and though comparatively 
shallow, corals were growing over a large part of it. Ee 

In the larger islands, the lagoons contain but small reefs 
compared with their whole extent; the greater part is an open 


STRUCTURE OF CORAL ISLANDS. 1838 


sea, with deep waters and a sandy or muddy bottom. | There 
are instances, as at the southern Maldives, of a depth of 50 
and 60 fathoms, From 20 to 35 fathoms is the usual depth 
in the Paumotus. This was the result of Captain Beechey’s 
investigations ; and those of the Expedition, though few, cor- 
respond.. It is however probable that deeper soundings would 
be found in the large island of Nairsa (Dean’s). In Gilbert’s 
Group, southeast of the Carolines, the depth, where examined 
by the Expedition, varied from 2 to 35 fathoms. Mr. Darwin 


found the latter depth at Keeling Island. Chamisso found 


25 to 35 fathoms at the Marshall Islands. 
The bottom of these large lagoons is very nearly uniform, 
varying but little except from the occasional abrupt shallow- 


ings produced by growing patches of reef. Soundings bring 





up sand, pebbles, shells, and coral mud; and the last men- 
tioned material appears to be quite common, even in lagoons 
of considerable size. It has the same character as above de- 
may be classed with these deposits. Darwin describes this mud 
as occurring at the Maldives, and at Keeling Island (op. cit. 
p. 26); Kotzebue mentions it as common at the Marshall 
atolls, and Lieutenant Nelson observed it at the Bermidas. 
It appears, therefore, that the finer coral material of the 
shores prevails throughout the depths of the lagoon. The 
growing reefs within the lagoons are in the condition of the 
inner reefs about high islands. The corals grow but little 
disturbed by the waves, and the reef-rock often contains them 
in the position of growth. At Taputeouea (Kingsmill’s or 
Gilbert's Group), reefs very similar to those of the Feejees 
occur; they contain similar large Astreeas ten to twelve feet 
in diameter, which once were growing where they stand, but 
are now a part of the solid lifeless rock. 


184 CORALS AND CORAL ISLANDS. 


Beach formations of coral sand-rock are common on the 
coral islands, and they present the same features in every re- 
spect as those described. They were observed among the Pau- 
motus, on Raraka, Honden, Kawehe, and other islands. The 
stratified character is always distinct, and the layers slope to- 
ward the water at the usual small angle, amounting to 5-7 
degrees bordering the lagoon, and 6—8 degrees on the seashore 
side of the land. Agassiz gives the same angle for the sea- 
ward slope of similar deposits at Key West. The rock is 
largely a fine odlite. They often occupy a breadth of thirty 
to fifty yards, appearing like a series of outcrops; yet they are 
frequently covered by the sands of the steep part of the beach. 
The rock is a fine or coarse sand-rock, or odlite, or coral 
pudding-stone, and consists of beach materials. Occasion- 
ally it is compact, and resembles common limestone, except- 
ing in its whiter color; but generally its sand origin is 
apparent. Deposits of sand and fragments of corals and 
shells cover the top of a reef-bank underneath what there 
is of soil; and they are horizontal instead of having the dip 
of the beach. 

In borings by Lieutenant Johnson, of the Wilkes Explor- 
ing Expedition, on Aratica or Carlshoff’s Island, in the Pau- 
motus, ten or eleven feet were passed through easily, and then 
there was a sudden transition from this softer rock (probably 
the beach sand-rock), to the solid reef-rock. 

The drift sand-rock was not met with by the author on any of 
the coral islands visited. The time for exploration on these 
islands allowed by the Expedition was too short for thorough 
work. It has been stated that the more exposed points toward 
the trades, especially the northeastern and southwestern, are 
commonly a little higher than other parts; and it is altogether 
probable that some of the sand heaps there formed will prove 


STRUCTURE OF CORAL ISLANDS. 185 


on examination to afford examples of this variety of coral- 
rock. Such situations are identical with those on Oahu, 
where they occur on so remarkable a scale. 

In the Atlantic, on the Bermudas and Bahamas, and Flor- 
ida reefs, the drift-sand accumulations often have heights of 
forty to one hundred feet and sometimes of one hundred and 
fifty to two hundred and fifty feet. They make the dry land 
or islands and cays of these regions as described beyond. 


Although in these descriptions of atolls, some points have 
been dwelt upon more at length than in the description of 
barrier reefs, still it will be observed that the former have no 
essential peculiarities of structure apart from such as necessar- 
ily arise from the absence of high rocky lands. The encircling 
atoll reef corresponds with the outer reefs that enclose hich 
islands; and the green islands and the beach formations, in the 
two cases, originate in the same manner. 

The lagoons, moreover, are similar in character and posi- 
tion to the inner channels within barrier reefs; they receive 
coral material only from the action of degrading agents, be- 
cause no other source of detritus but the reefs is at hand. The 
accumulations going on within them are, therefore, wholly of 
coral. ‘The reefs within the lagoons correspond very exactly 
in mode of growth and other characters to the nner reefs un- 
der the lee of a barrier. 


IV. NOTICES OF SOME CORAL ISLANDS. 


The preceding descriptions represent the general character 
of atolls, but are more especially drawn from the Paumotus. 
There are some peculiarities in other seas, to which we may 
briefly allude. 

Among the scattered coral islands north of the Samoan 


186 CORALS AND CORAL ISLANDS. 


Group, the shore platform is seldom as extensive as at the 
Paumotus. It rarely exceeds fifty yards in width, and is cut 
up by passages often reaching almost to the beach. In some 
places the platform is broken into islets. Enderbury’s Island 
is one of the number to which this description applies. The 
beach is eleven or twelve feet high. or the first eight feet it 
slopes very regularly at an angle of thirty to thirty-five degrees, 
and consists of sand, coarse pebbles, or rounded stones of coral, 
with some shells; and there is the usual beach conglomerate 
near the water’s edge. After this first slope, it is horizontal 
for eighty to two hundred feet, and then there is a gradual 
rise of three to four feet. Over this portion there are large 
slabs of the beach conglomerate, along with masses from the 
reef-rock, and some thick plates of a huge foliaceous Madre- 
pora; and these slabs, many of which are six feet square, lie 
inclining quite regularly against one another, as if they had 
been taken up and laid there by hand. They incline in the 
same direction with the slope of the beach. The large Madre- 
pora alluded to has the mode of growth of the Madrepora 
palmata ; and probably the entire zodphyte extended over 
an area twelve or fifteen feet in diameter. The fragments 
are three to four inches thick, and thirty square feet in surface. 

As a key to the explanation of the peculiarities here ob- 
served, it may be remarked that the tides in the Paumotus are 
two to three feet, and about Enderby’s Island five to six feet 
in height. 

Maldive Archipelago.—The Maldives have been often 
appealed to in illustration of coral structures. They are par- 
ticularly described by Mr. Darwin from information commu- 
nicated to him by Captain Moresby, and from the charts of this 
officer and Lieutenant Powell. A paper onthe northern Mal- 
dives, by Captain Moresby, is contained in the Journal of the 





Ly Se 








Phoowa 
gies 


QF) Addoo Atoll 


MALDIVE ARCHIPELAGO. 
One urch to CO mites. 


1 Ee } yr 


Plate X. 


° 


rr Whe ie 










_ MCZ LIBRARY 
) HARVARD UNIVERSHY — = fie 
CAMBRIDGE. MA USA . 





.s. ' 


STRUCTURE OF CORAL ISLANDS. 189 


Royal Geographical Society, vol. v., p. 398; and another on 
this group by J. J. Horsburgh, and W. F. W. Owen, in the 
the same journal, vol. ii, pp. 72 and 81. As stated by Mr. 
Darwin, the archipelago has a length of 470 miles, with an 
average breadth of 50 miles; and it consists for the most of 
its length of two parailel lines of atolls. The large atoll at 
the north end has a length of 88 miles, while Suadiva, one of 
the southernmost, is 44 miles long from north to south, and 
34 miles across. 

The point of special interest in their structure is the oc- 


currence of atolls or annular reefs within the larger atolls. 
yy Powells Is* 





MAHLOS MAHDOO ATOLL, WITH HORSBURGH ATOLL. 
Scale 1-20 of an inch to a mile. 


The islets of the lagoon and those of the encircling reef, are 
in many instances annular reefs, each with its own little lake. 
Gems within gems are here clustered together. 


190 CORALS AND CORAL ISLANDS. 


This feature is well exhibited in the Mahlos Mahdoo atoll, 
an enlarged map of which, from Darwin’s work, is here in- 
serted. The atoll consists of three main atoll-shaped portions ; 
but in each of these, the border is made up in part of atolls. 
Many of the subordinate atolls of the border are “three, and 
some even five miles in diameter, while those within the lagoon 
are usually smaller, few being more than two miles across, and 
the greater number less than one. The depth of the little 
lagoons within these small annular reefs is generally from five 
to seven fathoms, but occasionally more; and in Ari atoll, 
many of the central ones are twelve, and some even more than 
twelve fathoms deep. ‘These subordinate atolls rise abruptly | 
from the platform or bank on which they stand, with their 
outer margin bordered by living corals.” ‘The small atolls | 
of the border, even where most perfect and standing farthest 
apart, generally have their longest axis directed in the line 
which the reef would have held if the atoll had been bounded 
by an ordinary wall.” (Darwin, on Coral Reefs, pp. 33, 34.) 

The Maldives are among the largest atoll reefs known; 
and they are intersected by many large open channels; and 
Mr. Darwin observes, that the interior atolls occur only near 
these channels, where the sea has free access. We may view 
each large island in the archipelago as a sub-archipelago of 
itself. Although thus singular in their features, they illus- 
trate no new principles with regard to reef-formations. 

Mr. Darwin thus remarks (Op. cit. pp. 33, 34),—“ TI can 
in fact poimt out no essential difference between these little 
ring-formed reefs (which, however, are larger, and contain 
deeper lagoons than many true atolls that stand in the open 
sea), and the most perfectly characterized atolls, excepting 
that the ring-formed reefs are based on a shallow foundation 
instead of on the floor of the open sea; and that instead of being 


STRUCTURE OF CORAL ISLANDS. 19] 


scattered irregularly, they are grouped closely together.” —‘ It 
appears from the charts on a large scale, that the ring-like 
structure is contingent on the marginal channels. or breaches 
being wide, and, consequently, on the whole interior of the 
atoll being freely exposed to the waters of the open sea. 
When the channels are narrow, or few in number, although 
the lagoon be of great size and depth (as in Suadiva), there 
are no ring-formed reefs; where the channels are somewhat 
broader, the marginal portions of reef, and especially those 
close to the larger channels, are ring-formed, but the central 
ones are not so: where they are broadest, almost every reef 
throughout the atoll is more or less perfectly ring-formed. A1- 
though their presence is thus contingent on the openness of 
the marginal channels, the theory of their formation, as we 





GREAT CHAGOS BANE, 


shall hereafter see, is included in that of the parent atolls, of 
which they form the separate portions.” 


192 CORALS AND CORAL ISLANDS. 


The Great Chagos Bank, This bank lies about ten degrees 
south of the Maldives, and is ninety miles long and seventy in 
its greatest breadth. It is a part of the Chagos Group, in which 
there are some true atolls, some bare atoll-reefs, and others, like 
the Great Chagos Bank, that are quite submerged, or nearly 
so. Its rim is mostly from four to ten fathoms under water. 

Mr. Darwin confirms the opinion of Captain Moresby, that 
this bank has the character of a lagoon reef, resembling one 
of the Maldives; and he states, on the evidence of extensive 
soundings, that, if raised to the surface, it would actually be- 
come a coral island, with a lagoon forty fathoms deep. He 
says that, in the words of Captain Moresby, it is in truth 
“nothing more than a half-drowned atoll.” 

The form of the bank, its margin of shoals, and a line 
of soundings across it, giving the depth of the central or 
lagoon portion, are shown in the map on p. 191, from Darwin, 
and for which, as well as for other information about the 


4to 10 fathoms. 


2POr ms 


Leveh of the Sea. 


EOS HEL 


J \bo 
( 


Poet 


76 miles in length- 


EAST AND WEST SECTION ACROSS THE GREAT CHAGOS BANK. 


bank, he gives credit to Captain Moresby. ‘The cross section 
is still further illustrated in the annexed cut. The whole 
length of the section (or width of the bank in the line of .the 
soundings) is seventy-six miles. From the outer rim of 
the submerged atoll, there is a drop off to a deeper level, which 
is mostly fifteen to eighteen fathoms below the surface ; and 


STRUCTURE OF CORAL ISLANDS. 193 


then to the bottom of what was once the lagoon, now for 
the most part forty to fifty fathoms under water, though hay- 
ing its shoals that are five to ten fathoms submerged. All 
points in the map that are shaded, have a depth of less than 
ten fathoms; the only emerged parts are three or four spots 
on the western margin, as indicated on the map above. The 
bottom over the interior is muddy; on the flat bordering it, 
15 to 20 fathoms deep, there is coral sand with ‘‘a very little 
live coral; the outer rim is coral rock with scarcely any live 


” while the shoals or knolls of the interior are ‘ cov- 


coral ; 
ered with luxuriantly-growing corals.” Darwin states also 
that the rim is steep on both sides, and outward slopes abruptly 
to unfathomable depths; at a distance of less than half a mile 
from one part no bottom was found with 190 fathoms; and 
off another point, at a somewhat greater distance, there was 
none with 210 fathoms. For other similar facts see page 
Metia and other elevated Coral Islands.—Metia, or Au- 
rora Island, is one of the western Paumotus. It is a small 
| island about four miles by two and a half in width, and two 


, hundred and fifty feet in height; and it consists throughout 













































































































































































































































































METIA, OR AURORA ISLAND 


of coral limestone. Approached from the northeast, its high 


vertical cliffs looked as if basaltic, resembling somewhat the 
13 


194 CORALS AND CORAL ISLANDS. 


Palisades on the Hudson. This appearance of a vertical 
structure was afterward traced to vertical furrowings by the 
waters dripping down its front, and the consequent formation 
of stalagmitic incrustations. Deep caverns were also seen. 

The cliff, though vertical in some parts, is roughly sloping 
in others, and on the west side, the surface of the island grad- 
ually declines to the sea. 

The rock is a white and solid limestone, seldom presenting 
any traces of its coral origin. In some few layers there were 
disseminated corals, looking like imbedded fossils, along with 
beautiful casts of shells; but for the most part it was as com- 
pact as any ancient limestone, and as uniform in texture. Oc- 
casionally there were disseminated spots of crystallized calcite. 

The caverns contain coarse stalactites, some of which are 
six feet in diameter; and interesting specimens were obtained 
containing recent land shells that had been enclosed by a cal- 
eareous film while hibernating. | 

It is probable that more extensive caverns would have 
been found had there been more than a few hours for the ex- 
amination of the island. The Rey. Mr. Williams, in his work 
on Missionary Enterprises in the Pacific, gives very interesting 
descriptions of caverns in the elevated coral rock of Atiu, 
one of the Hervey Group. In one, he wandered two hours, 
without finding a termination to its windings, passing through 
chambers with ‘“ fretwork ceilings of stalagmite and stalactite 
columns, which, ’mid the darkness, sparkled brilliantly with 
the reflected torch-light.” This author remarks, ‘‘ that while 
the madrepores, the brain and every other species of coral are 
full of little cells, these islands (including those resembling 
Atiu), appear to be solid masses of compact limestone, in 
which nothing like a cell can be detected.” 

Beechey, in his description of Henderson Island, another 


STRUCTURE OF CORAL ISLANDS. 195 


of this character, speaks of the rock as compact, and having 
the fracture of a secondary limestone. 

The surface of Metia is singularly rough, owing to ero- 
sion by the rains. The paths that cross it wind through nar- 
row passages among ragged needles and ridges of rock as high 
as the head, the peaks and narrow defiles forming a miniature 
model of the grandest Alpine scenery. ‘There is but little 
soil, yet the island is covered with trees and shrubbery. 

The shores at the first elevation of the island, must have 
been worn away to a large extent by the sea; and the cliff 
and some isolated pinnacles of coral rock still standing on the 
coast are evidence of the degradation. But at present there 
is a wide shore-platform of coral reef, two hundred or two 
hundred and fifty feet wide, resembling that of the low coral 
islands, and having growing coral, as usual, about its margin 
and in the shallow depths beyond. 

In the face of the cliff there are two horizontal lines, 
along which cavities or caverns are most frequent, which con- 
sequently give an appearance of stratification to the rock, 
dividing it into three nearly equal layers. 


We might continue this account of coral reefs and islands, 
by particular descriptions of others in the Pacific. But the 
similarity among them is so great, and their peculiarities are 
already so fully detailed, that this would amount only to a 
succession of repetitions. The characters of a few, briefly 
stated, will suffice in this place. The first eight mentioned 
beyond ‘are small islands situated within ten degrees of the 
equator; Birnie’s, Enderbury’s, and Hull’s belonging to the 
Pheenix Group, and Oatafu and Fakaafo to the Union Group. 
Their positions are shown on Plate VIILf. The remainder are “7Y~ e 


9° 


a few islands of the Paumotu Archipelago, in latitudes 12 


196 CORALS AND CORAL ISLANDS. 


to 21° 8. Several of them are guano islands, as described 
on pages 318 to 324. 

Birnie’s. — Lat. 3° 35’ 8. Long. 171° 30’ W.  Four- 
fifths of a mile by one-third, trending northwest. No lagoon. 
A sandy flat about ten feet high, except near the north-north- 
east extremity, where it is about twelve feet. To the south- 
southwest the submerged reef extends out nearly a mile, over 
which the sea breaks. In passing it, distinguished no vegeta- 
tion except the low purslane and some trailing plants. 

Enderbury’s. — 3° 8S. 171° 15’ W. 4 miles by 1 mile 
nearly, trending N. N. W., and 8. 8. E.; form trapezoidal or 
nearly rectangular. Little vegetation on any part, and but 
few trees. The lagoon very shallow, and containing no grow- 
ing coral; its shores of coral mud, allowing the foot to sk in 
eight or ten inches, and covered in places with saline incrus- 
tations. Shore platform one hundred feet or less in width, 
and surface inclined outward at a very small angle; covered 
with three or.four feet of water at high tide, and with few 
corals or shells; beyond this, falls off four to six feet, and then 
the bottom inclines for one hundred yards or more. The beach 
very high and regular; rises eight feet at an inclination of 
thirty to thirty-five degrees ; then horizontal for eighty to two 
hundred, after which another rise of three or four feet. It 
consists of pebbles and fine sand, but above of slabs and 
blocks of coral rock and of the beach sand-rock, those of the 
latter nearly rectangular and flat. This beach sand-rock 
occurs in layers from ten to twenty inches thick along the 
shore, and is inclined from five to seven degrees seaward. 
Some portions are very compact, and ring under the ham- 
mer, while others enclose fragments of different sizes to a 
foot or more in diameter. Large trunks of transported trees 
lay upon the island, one of which was forty feet long and 


STRUCTURE OF CORAL ISLANDS. 197 


four in diameter. The shore platform was much intersected 
by channels. 

Captain Hudson obtained soundings half a mile off in two 
hundred fathoms; the lead struck upon a sandy bottom, but 
was indented by coral. | 

Hull’s Island. — Lat. 4° 20’ 8. Long. 171° 15’ W. 
Trends northeast and southwest. Well wooded nearly all 
round ; but on leeward side the forest in patches, with breaks 
of bare coral. Lagoon narrow, without entrance. Width of 
island from sea to lagoon, one hundred to four hundred yards: 
width greatest at south end. Beach ten feet high. The soil 
of the island consisted of coral fragments and sand. Shore 
platform fifty to eighty feet wide; five or six feet of water 
over it at high tide. Cut up very irregularly by channels 
three to eight or ten feet wide. Observed small corals grow- 
ing on the bottom outside of the platform. Shores of lagoon 
shallow for fifty yards, and consisting of coral sand. Beyond 
this a slope covered with growing corals; the corals rather 
tender species of Madrepores. In the interior of the lagoon 
many knolls and large patches of coral. 

Swain’s. — (Fig. 8, page 168.) Lat. 11° 10’ S. Long. 
170° 52’ W. 1:3 miles by 2; shape nearly rectangular; 
trends east and west. No lagoon, but the centre a little 
lower than the sides. Surface covered with shrubbery and 
large trees, among the latter many cocoanuts; the centre 
more sparsely wooded. Height fifteen to eighteen feet, ex- 
_cepting on the middle of the western side, where the surface 
is covered with loose fragments of coral of small size; there 
appears to have been a former entrance to the lagoon at this 
place. Shore reef, or platform, one hundred yards in average 
width, and one hundred and fifty yards at the place where 
we landed. Beach high, ten to twelve feet. At lower part 


198 CORALS AND CORAL ISLANDS. 


of beach, for a height of two to three feet, the coral reef-rock 
was exposed, indicating an elevation of the island. For three 
or four feet above this, layers of the beach sand-rock were 
often in view, consisting of coral pebbles firmly cemented, and 
having the usual dip of seven or eight degrees seaward ; in 
many places it was concealed by the beach sands and pebbles. 
There was no growing coral on the platform, excepting Nulli- 
pores. The outer margin of this platform was very uneven, 
and much intersected by channels, though less so than at 
Enderbury’s Island. Great numbers of Birgi (large Crustacea) 
were burrowing over the island, some of which were six 
inches in breadth. 

Oatafu or Duke of York’s. — 8° 38'S. 172°27'W. Form 
oblong, trending northwest. Length 33 miles; breadth 2 miles. 
Circuit 94 miles, and about one-half wooded in patches. South- 
west reef mostly bare. A lagoon, but without entrance except 
for canoes at high tide, on leeward side. Island ten feet high. 
Shore platform narrow, and intersected by channels. Shores 
lined by reef-rock, two or three feet out of water, indicating 
an elevation of the island. This reef-rock consists of various 
corals firmly cemented. Within the lagoon, knolls of coral, 
but none near the shore on the leeward side. 

Fakaafo or Bowditch’s. — 9° 20'S. 171° 5’ W. 63 miles 
by 4. Shape nearly triangular. Circuit seventeen miles, 
about six of which are wooded in several patches, separated 
by long bare intervals. A large lagoon, but no ship entrance. 
Height of island, fifteen feet. Width to the lagoon, one hun- 
dred to two hundred yards. Soil of the island coral sand, 
speckled black with results of vegetable decomposition. Shore 
platform narrow. At outer edge a depth of three fathoms, 
and from thence gradually deepens, and abounds in corals 
for fifty yards, when it deepens abruptly. Coral reef-rock 


STRUCTURE OF CORAL ISLANDS. 199 


elevated three or four feet, mdicating an elevation of the 
island. . Lagoon shallow, with some growing coral, but none 
near the shore. Some corals growing on the platform, near 
its margin, mostly small Madrepores, Astraas, Nullipores. 
Fragments of pumice were found among the natives, which 
had floated to the island (see fig. 5, page 168). 

Washington Island. — Lat. 4° 41’ N. Long. 160° 15’ W. 
3 miles by 14, trending east and west. It is a dense cocoanut 
grove with luxuriant shrubbery. No lagoon. The shore plat- 
form is rather narrow. <A point of submerged reef, one and a 
half miles long, stretches out from the southwest end. Did 
not land on account of bad weather. 

Otuhu, Paumotu Archipelago. — 14° 5’ 8S. 141° 30’ W. 
1i miles by 2, trending north and south. No lagoon. 
Wooded. 

Margaret, Paumotu Archipelago. — 20° 42'S. 143° 4’ W. 
Diameter one mile, nearly circular. A small shallow lagoon 
with no entrance. Northeast side alone wooded, and in two 
patches. | 

Teku or Four Crowns, Paumotu Archipelago. — 20° 28'S. 
143° 18’ W. Diameter 14 miles, nearly circular. <A small 
lagoon with no entrance. Southwestern reef bare; five 
patches of forest on the other part. 

Honden or Henuake, Paumotu Archipelago. — Size 3! 
miles by 2 miles. Oblong, five-sided; trending west-north- 
west. A small, shallow lagoon, communicating with the sea 
only at high tide, on the west side. There are two other 
entrances which are seldom if ever covered with water, and 
appeared merely as dry beds of coral rock. Height of the 
island twelve feet ; lowest on the south side. Belt of verdure 
complete, and consisting of large forest trees, with the Pan- 


danus and other species, but no cocoanuts; its breadth half a 


200 CORALS AND CORAL ISLANDS. 


mile, and in some parts three-fourths. Among the trees large 
masses of coral rock often exposed to view, and the surface in 
many parts very rough. It seemed surprising at all these 
islands that there should be so luxuriant a growth of trees 
and shrubbery over so rocky a surface. Shores of the lagoon 
nearly flat. On one side there was a large area of extremely 
fine coral sand and mud, which extended a long distance into 
the lagoon. Elsewhere about the centre of the island the 
reef-rock was bare, and contained numerous shells of Tri- 
dacnz. A few small Madrepores still growing in the lagoon. 
Beach on the sea-shore side eight feet high. In lower part 
of beach several layers of white limestone (the beach sand- 
rock), formed of coral fragments or sand, shells, etc., much 
of which was very compact. The layers inclined toward the 
sea at an angle of about six degrees. Shore platform as else- 
where in this archipelago. 

The facts above stated are evidence of a slight ele- 
vation, probably not exceeding three feet (see fig. 5, page 
168). . 

Taiara, or King’s, Paumotu Archipelago. — 15° 42’ S. 
144° 46’ W. 22 miles by 13, trending northwest. Has a 
small lagoon with no entrance. Reef almost continuously 
wooded around, somewhat broken into patches. 

Ahii, or Peacock’s Island, Paumotu Archipelago. — 14° 
30’ S. 146° 20’ W. 13 miles by 6, trending N. E. by EH. 
Shape irregularly oblong. A large lagoon, having an entrance 
for small vessels on the west. Reef wooded throughout nearly 
its whole circuit. Lagoon shallow, and much obstructed by 
growing coral, the latter giving the water over it a clear light 
green color. Platform, or outer coral shelf of the island, 
about two hundred and fifty feet wide; under water except 
at the lowest tides. Margin highest, and covered with Nulli- 


STRUCTURE OF CORAL ISLANDS. 201 


pore incrustations, which give it a variety of delicate shades of 
color, mostly reddish, of peach-blossom red, rose, scarlet. For 
thirty to fifty feet from the margin, very cavernous, and con 
taining many Tridacne, lying half imbedded, with the variously 
tinted mantle expanded when the surface is covered with wa- 
ter. Rock of the platform either a compact white limestone or 
a solid conglomerate; dead over its surface, excepting a few 
Madrepore tufts or Astreas near the margin in pools. In this 
shelf there were long fissures, extending nearly parallel with the 
shore, a quarter to half an inch wide at top, and continuing 
sometimes a fourth of a mile or more. These fissures were com- 
monly filled with coral sand. ‘The higher parts of the island 
either consisted of loose blocks of coral or were covered with 
some soil ; the soil mostly of comminuted coral and shells, with 
dark particles from vegetable decomposition intermingled. On 
the bottom, exterior to the shore platform, observed the same 
corals growing as occurred in fragments upon the island ; but 
the larger part of the bottom was without coral, or consisted 
only of sand. 

Raraka, Paumotu Archipelago.—16° 10’S., 145° W. 14 
miles by 8, trending east and west. Shape somewhat trian- 
_ gular. North side nearly continuously wooded; south angle 
and southwest reef bare. A large lagoon with an entrance 
for small vessels on the north side. A rapid current flows 
from the entrance, which it was difficult for a boat to pull 
against. Shore platform, as usual, about a hundred yards 
wide, with the edge rather higher than the surface back ; the 
platform mostly bare of water at low tide. Several large 
masses of coral and coral rock, one to four hundred cubic feet, 
on the platform and upon the higher parts of the island, some 
of which stood five and six feet above high-water mark; they 
were cemented to the reef-rock below, and appeared like project- 


202 CORALS AND CORAL ISLANDS. 


ing parts of the reef. Layers of beach sand-rock on the lagoon 
shores, as well as on the seaward side, inclined at an angle of 
six or seven degrees: characters as already described. Grow- 
ing coral in the entrance to the lagoon, within two feet of the 
surface, mostly a species of Millepora (M. squarrosa). Inte- 
rior of the lagoon not examined, no time being allowed for it by 
the Expedition. The water looked as blue as the ocean, and 
was much roughened by the winds. 

Kawehe or Vincennes Island, Paumotu Archipelago, 15° 
30’ S., 145° 10’ W. 13 miles by 9, trending north-north- 
west. Shape irregularly oval. Having a large lagoon, and 
mostly wooded around, least so to leeward. Between the 
wooded islets (as on, Raraka and elsewhere), surface consisted 
of angular masses of coral rock (among which the Porites pre- 
vail), strewed in great numbers together; and in some parts 
bearing a few vines and purslane among the blocks, though 
scarcely any appearance of soil, or even of coral sand. In 
other parts, not as high, no vegetation, and surface still wet 
by high tide. A few large masses of coral on the shore plat- 
form, either lying loose, or firmly attached below, as already 
described; some of them were six feet cube, and one was 
raised seven feet above high-water mark. Shore platform about 
a hundred yards wide, rather highest at the edge, and much of 
its surface two to four feet under water at low tide. As else- 
where, this platform is nothing but a compact coral conglom- 
erate or limestone, having no growing coral over it, except in 
some shallow pools near its outer margin, where also there 
are numerous holes in which crabs are concealed, with small 
fish and other animals of the shores. On the lagoon shore, 
layers of beach sand-rock, six or seven in number, dipping at 
an angle of seven degrees toward the lagoon, and outcropping 
one above the other. Similar layers on the sea-shore side 


STRUCTURE OF OORAL ISLANDS. 203 


Manhii, Wilson’s or Waterlandt, Paumotu Archipelago, 
14° 25’ S., 146° W. 15 miles by 6, trending EK. N. EK. A 
large lagoon with a deep entrance on the west side. Shape 
oblong triangular. 

Shore platform as usual; mostly under water at low tide. 
Large masses of coral here and there, standing on this reef, 
either cemented to it, or loose. One top-shaped mass is figured 
on p.179. High water did not reach the part of it which was 
most worn ; and this was evidently owing to the fact that the 
action of the swell or waves is greatest above the actual level 
of the tide at the time. The reef-rock is either a compact 
limestone, showing no traces of its composite origin, or a con- 
glomerate. Beach, regular as usual, six to ten feet high, con- 
sisting of coral sand, and fragments of worn shells, with occa- 
sional exuvie of crabs, remains of Echini, fish, etc. The en- 
trance to the lagoon is deep and narrow, with vertical sides. 

Aratica or Carlshof, Paumotu Archipelago, 15° 30’S., 
145° 30’ W. 17 miles by 10, trending N. E: Large lagoon 
with a good entrance for vessels. The reef fronting south 
bare for nine miles; on northwest side, mostly very low, with 
only here and there a clump of trees; occasionally a line of 
wooded land for a quarter of a mile on the east side; more 
continuously wooded on the north. The bare parts mostly 
covered with blocks of coral, one to thirty cubic feet and larg- 
er, tumbled together as on the preceding. Some blocks of 
coral on the shore platform very large; one eight feet high 
and fifteen in diameter, containing at least 1,000 cubic feet. 

Nairsa or Dean’s, Paumotu Archipelago, 15° S., 148° W. 
44 miles by 17, trending W. N. W. Northern shore mostly 
wooded; southern with only an occasional islet, connected by 
long lines of bare reef. In these intervals, the reef stood 
eight feet or so out of water, according to estimate from ship- 


204 CORALS AND CORAL ISLANDS. 


board, and was worn into a range of columns, or excavated 
with caverns, so as to look very much broken, though quite 
regularly even in the level of the top line. 


We continue these descriptions with notes on some coral 
reefs and islands of the Atlantic, derived from the charts of 
the ocean and different publications mentioned beyond. 

Florida Reefs. — The position of the Florida coral reefs, 
and of the Bahama Islands with reference to Florida and 
Cuba, and to one another, and the depths over and among 
the reefs, are shown on Plate XI., which is taken from one 
of the Atlantic Ocean charts of the United States Hydro- 
graphic Department.’ 

The Florida coral formations are mainly great reef-made 
banks. They commence on the east coast, at Cape Florida, 
in latitude 25° 40’ N., continue around the south extremity 
of the peninsula, and stretch westward to and beyond Key 


1 The chart is Sheet One of the North Atlantic Ocean, Lower Part (No. 21). 
It has great geological interest; for it includes all the West Indian and Gulf seas, 
with also the Bermudas and northern South America to the equator, and gives all 
soundings to date of publication. This chart and another of the Florida reefs and 
Bahamas on a still larger scale (No. 944) are the best sources of information as to 
the extent, positions, forms, general characters, and relations of the reef regions, 
and the distributions of islands and keys; and detailed descriptions are of little 
value without them. 

The more important sciéntific accounts of the Florida reefs are the following: 
Prof. Louis Agassiz’s Memoir, published, in part, in the Coast Survey Reports for 
1851, and entire, with added illustrations of West India corals, in the Memoirs of 
the Museum of Comparative Zodlogy, vol. vii. 1880; M. Tuomey’s paper in the 
American Journal of Science, 1851, 2d ser., xi.; Publications of F. de Pourtales, in 
the Bulletin of the Museum of Comparative Zodlogy for 1867, 1868, 1878, and his 
Report on Deep-Sea Corals in the Illustrated Catalogue of the Museum, in 1871; the 
excellent and finely illustrated work of Mr. Alexander Agassiz, entitled the “ Three 
Cruises of the Blake,” in two volumes, 1888; besides papers by Mr. Agassiz in the 
Bulletin of the Museum of Comparative Zoology, and in the Memoirs of the American 
Academy, 1883, xi. Other papers are those of Prof. Joseph Le Conte, American 
Journal of Science for 1857, xxiii., 46, on the Agency of the Gulf Stream in the For- 
mation of the Peninsula and Keys of Florida; and Capt. E. B. Hunt, U.S. A., ibid., 
1863, xxxv., 197, on the Origin, Growth, and Chronology of the Florida Reef. 


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FLORIDA BANKS. 205 


West. They are broad reef-ground flats more or less swept 
by the tides; the Biscayne Bay, two to eight miles wide, is 
on the east side of Florida, and the larger Florida Bay on the 
south. The width of the bank rapidly increases to the west- 
ward, becoming thirty miles off Cape Sable. No subdivision 
into a fringing and barrier reef is suggested by its features. 
“The whole tract between Cape Sable and the Keys as far as 
Cape Florida, at least as far as Soldier Key, is so shoal that 
it is inaccessible except for very small vessels.” This shoal 
region, as Professor Agassiz’s Memoir (1851) states, is literally 
studded with mangrove islands, either In continuous ranges 
of great extent, or making “innumerable small islets, so inti- 
mately interwoven and separated by so narrow and shallow 
channels as to be almost impenetrable.” The mangrove 
bushes, which seem to be “striding out in the mud” in conse- 
quence of the many root-making stems that grow downward 
from the branches, serve to entangle the sand, sea-weed, and 
drift-wood that float by, and thus aid in the process of emer- 
gence. The material of the great mud-flats of the area on 
the bottom of the shoal water is the finest of coral sand; 
but it is darkened in color by the carbonaceous results of 
animal and vegetable decomposition, so that much of it looks 
like ordinary silt or mud. 

Along the western side of Florida the coast-flats extend 
from Cape Sable northward for fifty to sixty miles, with a 
mean width of nearly ten miles; but the sands of the flats 
are partly siliceous. Outside of the bank the waters are 
deeper, but the bottom continues to be largely calcareous.' 
The bed of coral rock at Tampa Bay containing silicified 
corals has been supposed to be of recent origin; but it is 


1 The “Three Cruises of the Blake,” of Mr. A. Agassiz, has, at page 286 of 
the first volume, a colored map of much interest illustrating the nature of the 
sea-bottom in the Florida region. 


206 CORALS AND CORAL ISLANDS. 


proved, by the observations of Dr. E. C. Stearns and Prof. A. 
Heilprin, to be Tertiary and probably Miocene. At Tampa 
Bay and along the coast south there is shell-rock, or coquina, 
in process of formation in some places. The large accumu- 
lations of worm-like tubes on this coast, which have been 
referred to Serpule, have been recently found by Mr. W. H. 
Dall to be the tubes of a Mollusk, — the Gastropod Vermetus 
nigricans, Whose worm-like spirals are much like those of 


sea-worms.! 


Along the outer borders of the great Florida Bank thea 


waters abound in growing corals. They cover its margin, 
and also the bottom adjoining at depths of one to seven or 
eight fathoms, this being the ordinary limit of reef-forming 
corals in the Florida seas. Over this marginal belt of grow- 
ing corals, toward its outer limit, there are many spots of 
half-emerged reef and some islets; and these reefs make the 
outer boundary of a navigable channel carrying four to seven 
fathoms of water. This belt of growing corals is ordinarily 
called the “ Florida Reefs.” 

The islands and keys of the bank are almost wholly of 
wind-drift origm. The sands of the outer margin of the reef, 
where thrown up by the waves, have been taken up by the 
winds and deposited in long, high drifts a mile or so back. 
Key Largo, one of the eastern of the drift ridges, is thirty 
miles long and between one and two miles wide. Key West 
is near the southwestern extremity of the bank, and has the 
rather unusual height of fifteen feet above tide-level. Hast 
of Key West for thirty-five miles the Keys are cut into strips 
that trend north and south, —the Pine Islands, — owing to 
passages that have been kept open by the tides. 

The drift-sands of the Keys are consolidated into a coral 


1 Dall, American Journal of Science, 1887, xxxiv., 161. 


| q 


\ 


— 


FLORIDA BANKS. 207 


sand rock, and much of it into true odlite. The manner in 
which this consolidation is produced is explained on page 153. 

Pourtalés was the first to point out that the sand-made 
reef off the coast of Northern Florida continues to consist of 
siliceous sands quite down to Cape Floridi; that abruptly at 
this pot it becomes a coral-made reef, and is so continued \ 
southward. The facts mdicate, as observed by Major E. ice 
Hunt in 1863, that the drifting action of currents parallel 
with the coast have had much to do in locating the coral- 
built reef as well as the banks of siliceous sands; the 
material supplied is the chief difference between the two, 
coral sands failmg where growing corals fail. Major Hunt 
pointed out also that the current along the Florida coral reefs 
was a current counter to the Gulf Stream. cape 

How far the encroachment of siliceous sediments from the _ 
north may have determined the limit of coral-growing in that 
direction has not been ascertained. / If, as the author states 
in his Wilkes Expedition Report on Crustacea, the isocryme of 
68° F. terminates at Cape Canaveral, it would appear that the 
coral seas, or those warm enough to grow corals, extend one 
hundred and seventy-five miles north of Cape Florida, and that 
the northern siliceous sands have encroached all this distance. 

The limit may have been even north of Cape Canaveral ; 
for, as the author has recently learned from Professor Verrill, 
corals of the genus Oculina have been dredged up from a 
rocky bottom off Charleston, South Carolina, in depths of 
eight to ten fathoms. There is a question, however, whether 
the Oculina was one of the reef-making species. 

Little is seen over the flats or reefs of the true coral-reef 
rock, or the under-water coral limestone, which is the main 
material of all coral formations. It is generally concealed 
beneath the drift-sands of the surface. Tuomey observes 


KK 


208 CORALS AND CORAL ISLANDS. 


that — “the greater number, if not all the Keys, rest upon a 
foundation of corals. At Sand Key large rugged masses of 
dead coral are seen bordering the Key on the windward side, 
and rising above low water; similar masses may be seen at 
Sambo Key, and at other places along the outer reef. But 
the Keys within this barrier present better opportunities for 
studying the foundation upon which they rest. At Key 
Vacca corals rise to a height of four feet above high water, 
and present not the slightest evidence of disturbance, beyond 
the upward movement which raised them to their present 
position. The rocky mass of coral along the margin of the 
Key is undermined by the waves, and otherwise worn into sin- 
gularly rugged shapes, with sharp projecting points. Even at 
some distance from the water, bunches of coral project above 
the surface wherever the overlying sand is washed away. 
“In the shallow water off many of the Keys very beauti- 
ful patches of Alga, interspersed with living corals, are seen 
within six or eight inches of the surface. Off Indian and 
Plantation Keys dark knobs of coral are visible upon the 
white mud of the bottom, which render the navigation 
amongst these Keys dangerous. On lower Matacumba I 
traced the rugged coral rocks for a mile in extent; I also 
found them on Conch Key, as I did indeed on nearly every 
island that I examined, where a section could be found on 
the shore, from which the overlymg sands were washed.” 
What is the actual thickness of the Florida coral-reef 
formation, and what it rests upon, no one knows, as no at- 
tempts to ascertain by borings have been made. The Missis- 
sippi silt makes no encroachments on the region; for it is 
carried westward by the tidal currents of the Gulf, and does 


~ not reach eastward to within five hundred miles of the Flor- 


ida reef region. 


FLORIDA BANKS. 209 


The banks continue’ westward of Key West to the Mar- 
quesas Key, which is atoll-like in form and probably in ori- 
gin. But it is so situated at present within the sand-made 
area that there are no growing corals “on its weather-side.” 
Farther west a true atoll—that of the Tortugas — stands 
apart from the bank, a channel of eleven to sixteen fathoms 
intervening between it and the Marquesas reef. Branching 
corals and others are growing in abundance about it, which 
are the source of the coral fragments that are thrown on the 
beach. The bank may perhaps receive some light calcareous 
silt from the reefs to the eastward ; but the intervening chan- 
nel through which the tide sweeps must be filled before drift- 
ings from the eastward will reach effectively the Tortugas. 

Whether a bend in the south end of the peninsula of 
Florida determined the direction of drift of the marine cur- 
rents in the region, and the bend in the form of the skirting 
reef, or whether the direction of current drift alone gave di- 
rection to the belt of reef, is not determined by any positive 
facts. The east-and-west trend of Cuba favors the former of 
these explanations; and the position of the Tortugas leads 
to the conclusion that it was probably an atoll off the ex- 
tremity of the Florida Bank. An artesian boring in the 
centre of the Tortugas with a diamond drill that would sup- 
ply a large core for study, would give the facts for positive 
conclusions as to the nature of the basement and the depth 
to which it consists of coral-reef rock. 

No good evidence of elevation of the Florida Bank has 
been reported. The height of the islands and cays is not 
satisfactory evidence, as it is so generally due to sand-drifts. 
At the same time the opinion that there had been a rise of 
half a dozen feet or so may be found to have much in its 


favor. 
14 


210 CORALS AND CORAL ISLANDS. 


Region between the Florida Reefs and Cuba.— Through 
the labors of Pourtalés, in connection with the soundings 
by the Coast Survey, interesting facts have been brought to 
light respecting the sea between the Florida reefs and the 
opposite shores or reefs along Cuba and the Bahamas (see 
Plate XI.). A few paragraphs on the region, by Pourtalés, 
are cited from his memoir.t 

“Tn transverse sections of the channel the greatest depth 
is nearest its southern or eastern shore, and in a longitudinal 
section the depth diminishes in passing toward the north, 
finding its minimum in the narrowest part between Cape 
Florida and the Bemini Islands, after which it increases 
again. In a transverse section between Key West and 
Havana, the greatest depth is 855 fathoms; between Som- 
brero Light and Elbow or Double-Headed Shot Key on the 
Cay Sal, or Salt Key Bank, 500 fathoms; between Carys- 
fort reef and Orange Key on the Great Bahama Bank, 475 
fathoms; and between Cape Florida and the Bemini Islands, 
370 fathoms.” 

Pourtalés says of the bottom outside of the Florida reefs : 

“‘ Although the deep blue color of the water after passing 
the reef seems to indicate a very abrupt slope, there is in no 
part of it anything to compare with the sudden deepening on 
the edge of the coral reefs of the Pacific Ocean, or even of 
the Bahamas or the coast of Cuba. The distance from the 
reef to the 100-fathom line is not less than three miles, and 
often as much as six. In this space the bottom consists of 
calcareous mud, and is not particularly rich in animal life. 
From ninety or a hundred fathoms to two hundred and fifty 
or three hundred, the bottom slopes rather gently in the 


1 Museum of Comparative Zodlogy, Illustrated Catalogue, Cambridge, Mass., 
1871. 


BETWEEN FLORIDA AND CUBA. 911 


shape of a rough, rocky floor, without great imequalities ; 
this formation obtains its greatest breadth, of about eighteen 
miles, a little to the east of Sombrero Light, and tapers off 
to the west, where it ends in about the same longitude as the 
end of the reef; toward the east and north it approaches 
nearer the reef, and ends gradually between Carysfort reef 
and Cape Florida. This bottom, which is called the ‘ Pourtalés 
Plateau’ in Professor Agassiz’s report, is very rich in deep-sea 
corals, and most of the species described in the memoir [of 
Pourtalés] were dredged on this ground. Outside of the 
rocky bottom the Globigerina mud prevails and fills the 
trough of the channel. 

“On the Cuba shore the bottom is rocky and the slope 
very abrupt, particularly for the first four or five hundred 
fathoms. Along the Salt Key and Bahama Banks the slope 
is also exceedingly abrupt, but the underlying rock is often 
covered with mud.” 

The rock of the bottom of the Pourtalés Plateau is a lime- 
stone made from the calcareous relics of the species (corals, 
shells, echinoderms, etc.) living at those depths; and it is 
still increasing, well exemplifying what limestone-making 
may go on below the limit of reef-making corals. Mr. A. 
Agassiz has a cut on page 287 of his “Cruises of the 
Blake” representing a specimen of the material. 

The Salt Key Bank is mostly a submerged atoll-like area, 
setenty miles long, having emerged islets and cays along its 
northeastern and northern sides, or those to windward, and 
falling off southward to a depth of four and a half to six 
and a half fathoms and then steeply to depths of two hun- 
dred and fifty fathoms and beyond to those of the channel 
along the north side of Cuba. 

Prof. L. Agassiz describes the banks and keys embraced 


a2, CORALS AND CORAL ISLANDS. 


between Double-Headed Shot Key, Salt Key, and Anguilla 
Key as a very mnstructive combination of the phenomena of 
building and destruction. He says: ‘* The whole group is a 
flat bank covered by four or five, and occasionally six, fathoms 
of water, with fine sandy bottom, evidently corals reduced to 
odlite, the grains, which are of various sizes, from fine powder 
to coarse sand, mingled with broken shells, among which a 
few living specimens are occasionally found. The margin of 


the Bank is encireled on several points by rocky ridges of 


the most diversified appearance, and at others edged by sand- 


dunes. A close examination and comparison of the different 
Keys show that these different formations are in fact linked 
together, and represent various stages of the accumulation, 
consolidation, and cementation of the same materials. On 
the flat top of the bank the loose materials are pounded 
down to fine sand; in course of time this sand is thrown up 
upon the shoalest portions of the Bank, and it is curious to 
notice that these shoalest parts are its very edge, along which 
corals have formed reefs which have become the basis of the 
dry banks. The foundation rock, as far as tide, wind, and 
wave may carry the coarser materials, consists of a conglom- 
eration of coarser odlitic grains, rounded fragments of corals, 
or broken shells, and even larger pieces of a variety of corals 
and conchs, all the species being those now found living upon 
the Bank, among which Strombus gigas is the most com- 
mon; besides that, Astrea [ Orbicella| annularis, Siderastrea 
siderea, and Maandrina mammosa prevail. The shells of 
Strombus are so common that they give great solidity and 
hardness to the rock. The stratification is somewhat irregu- 
lar, the beds slanting toward the sea at an angle of about 
seven degrees. Upon this foundation immense masses of 


Strombus, dead shells, and corals have been thrown in banks, 


a 


BETWEEN FLORIDA AND CUBA. Ds 


evidently the beginning of deposits similar to those already 
consolidated below; but there is this difference in their for- 
mation, namely, that while the foundation rock is slightly 
inclined, and never rises above the level of high water, the ac- 
cumulation of loose materials above water-level forms steeper 
banks, varying from fifteen to twenty and thirty degrees. In 
some localities broken shells prevail; in others, coarse and 
fine sand; and the ridges thus formed, evidently by the action 
of high waves, rise to about twelve and fifteen feet. This is 
evidently the foundation for the accumulation of finer sand 
driven by the wind over these ridges, and forming high sand- 
dunes, held together by a variety of plants, among which a 
trailing vine (Latatas littoralis), various grasses, and shrubs 
are the most conspicuous. These dunes rise to about twenty 
feet ; on their lee side and almost to their summits grows a 
little palmetto. The sand of the dunes is still loose, but here 
and there shows a tendency to incrustation at the surface. 
The slope of these dunes is rather steep, sometimes over 
thirty degrees, and steeper to the seaward than on the land- 
ward side. 

‘In the interior of Salt Key there 1s a pool of intensely 
salt water, the tint of which is pinkish or flesh-colored, owing 
to the accumulation of a small alga. When agitated by the 
wind, this pool is hedged all around by foam of the purest 
white, arising from the frothing of the viscous water. Along 
the edge the accumulation of this microscopic plant forms 
large cakes, not unlike decaying meat, and of a very offensive 
odor. The foundation rock of this Key is exactly like what 
Gressly described as the ‘facies corallien’ of the Jurassic 
formation; while the deposit in deep water, consisting chiefly 
of muddy lime particles, answers to his ‘facies vaseux.’ 

“ Double-Headed Shot Key is a long, crescent-shaped 


ANA: CORALS AND CORAL ISLANDS. 


ridge of rounded knolls, not unlike ‘roches moutonnées,’ at 
intervals interrupted by breaks, so that the whole looks like a 
dismantled wall, broken down here and there to the water’s 
edge. The whole ridge is composed of the finest odlite, pretty 
regularly stratified, but here and there like torrential depos- 
its; the stratification is more distinctly visible where the rocks 
‘have been weathered at the surface into those rugged and 
furrowed slopes familiarly known as ‘ karren’ in Switzerland. 
\ It is plain that we have here the same formation as on Salt 
Key, only older, with more thoroughly cemented materials.” 

The Bahama Islands. —The Bahamas are, like the Flor- 
ida reefs, great coral-made banks, having their emerged land 
in the form of islands and cays. These dry portions are 
situated mostly along the windward sides. To comprehend 
the relations of the Bahamas to atolls they should be com- 
pared with the Louisiade Group, Plate VII., and other broad 
combinations of barriers and atolls, although true barrier 
reefs are absent. They are more remarkable than the Flor- 
ida reefs for their drift-sand accumulations, their height be- 
ing sometimes two hundred to two hundred and thirty feet, 
though generally under one hundred feet. A large part of 
the banks, however, are in a lagoon condition, being covered 
with water at depths ordinarily of two to five fathoms, and 
the leeward margin is for the most part submerged. 

The group extends for six hundred miles through the seas 
north of Cuba and Hayti, with the Florida Straits — fifty miles 
in mean width and three to five hundred fathoms in depth — 
separating them from Florida. The two great western banks 
are the Little Bahama, or the northwestern, one hundred and 
fifty miles long, and the Great Bahama Bank, to the south 
and east, three hundred and twenty-five miles long. The 
latter is shaped like a letter 8, or a pothook, and occupies two 


THE BAHAMA ISLANDS. 215 


thirds of the whole Bahama area. Nassau, the principal sea- 
port of the group, is situated on the island of New Provi- 
dence, at the northwest angle of the second turn in the S. 

The Bahama group continues eastward, in a number of 
coral islands, to Turk’s Island, in longitude 71°-W.; but topo- 
graphically and geologically it extends two degrees farther 
west in a line of atolls and reefs to Navidad Island. The 
small map to the right on Plate XI. contains Turk’s Island 
and the reef islands east of it. Turk’s Island is situated near 
the southeast angle of Caicos Island. 

The western portion of the Bahamas is built up on the 
broad submarine plateau, five hundred to six hundred fathoms 
under water, that extends parallel with the continental coast- 
line from off North Carolina southward. The plateau in this . 
southern part has a breadth from east to west of about two 
hundred miles. More to the eastward along the Bahamas 
the water deepens, and at the same time the reef becomes 
narrower; and from the east end of the S stretches a line 
of independent coral islands, some of which are typical 
atolls. 

The peculiar form of the “ Great Bahama Bank” is partly 
due to the coalescence of separate islands with the main west- 
ern portion through coral growths. It probably secured thus 
the addition of Long Island Bank, Eleuthera Bank, and Cat 
Island; and almost certainly the last of the three, Cat Island, 
whose connection with the part next west is made by a nar- 
row submerged reef, fifty to seventy-five feet under water, the 
northern side of which pitches off at an angle of about forty- 
five degrees to a depth of fifty-five hundred feet. 

The S-like form is dependent also on the intrusion into 
the area of two deep and broad tongues of the ocean, one 


going in from the northward, commencing with two thou- 


216 CORALS AND CORAL ISLANDS. 


sand fathoms at its entrance, and having depths inside of 
one thousand to seven hundred and fifty fathoms; and the 
other, Exuma Sound, entering from the east, and having 
depths within of twelve hundred fathoms toward the en- 
trance, and eight hundred near the mner extremity. 
Beyond the Great Bank, eastward, the ocean’s depths and 
the abruptness of the submarine declivities increase rapidly. 
Even in the New Providence Channel, east of the Little Ba- 
hama Bank, there are depths of twenty-two hundred and 
seventy fathoms, over thirteen thousand five hundred feet. 
The north shore of Eleuthera Island has nearly this depth 
within seven miles, or thirty-seven thousand feet, of its emerged 
reef; the pitch, therefore, 1:2°75. San Salvador, east of Cat 
Island, has a depth of fourteen thousand feet within three 
miles of the emerged reef, a pitch down of about 1:1:13, and 
a depth of sixteen thousand six hundred and forty-four feet 
within ten miles, the pitch 1:3; and almost eighteen thou- 
sand feet within twenty-one miles. These steep northern un- 
der-water slopes are continued along the course of the group 
eastward. Besides this, the channel on the south side of the 
group, Which is twelve hundred to fifteen hundred feet deep 
against middle Cuba, is over thirteen thousand five hundred 
feet north of Hayti. The line of reef-islands beyond Turk’s 
Island eastward have less emerged land than those to the 
westward. The last, Navidad, is mostly submerged reef, and 
has depths of thirteen thousand five hundred feet within fif- 
teen miles on the east and south, and seventeen thousand 
five hundred within thirty-five miles to the east-northeast. 
These facts as to the depths and steep submarine declivi- 
ties along the coral-reef islands are as remarkable as any 
yet observed in the Pacific Ocean, even among its equatorial 


islands. 


' . 
j if} iv" a 
ad 


THE BAHAMA ISLANDS. 217 


The rock of the Bahama Islands is, like that of the 
Florida Keys, a coral-sand rock of wind-drift origin. It is 
generally a poor building stone because of the numerous 
sand-flaws. Its weight varies from sixty-five to one hundred 
and forty-five pounds per cubic foot. In some of the basins 
or lagoon-bottoms a chalk-like deposit occurs, and nowhere so 
extensively as along the western coast of Andros Island. The 
coarser fragments of corals are never found much beyond the 
surf range of high tide. Captain Nelson says that “the south 
side of Silver Cay and the beach extending westward from 
Nassau afford rolled blocks, pebbles, and sand derived from 
the more massive corals, mixed with remains of turtles, fish, 
crustaceans, echinoderms, and mollusks. On the beach be- 
tween Clifton Point and West Bay the shells of Strombus 
gigas more especially accompany the rolled corals. At Hast 
Point the sand is derived from corallines and nullipores ; 
the finer sand being often in approximately spherical grains, 
though not so perfectly as the White Cay, and between Ex- 
uma and Long Cay. The beach near Charlotteville Pomt con- 
sists principally of Lucina Pennsylvanica in various stages of 
comminution. At Six Hills (Caicos Group) the mass of conch 
shells (Strombus gigas) is so great and sufficiently cemented 
together as to form not only rock, but an island several hun- 
dred feet in length. Along the northwest beach at Gun Cay 
a hard, coarse, stratified rock is formed of conch shells and 
others, together with coral fragments.” 

“A counterfeit odlite occurs near White Cay, Exuma, and 
elsewhere, in which the spherules have been derived appar- 
ently from the stems of corallines.” On the larger islands 
the rocky surface of the hills is very thinly and partially cov- 


? 


ered with “red earth”? mixed in varying proportions with 


vegetable matter. There are many large caverns in the 


218 CORALS AND CORAL ISLANDS. 


group, and those of Long Cay and Rum Cay are described as 
equalling those of the Bermudas. 

The Bahamas differ from ordinary atolls more in the 
great size of the two western banks, and the wide distribu- 
tion and high heapings of the wind-drift deposits, than in any 
other characteristics. Some peculiarities are due also to the 
position of the reefs, — one end resting on a sea-border pla- 
teau and the other extending out into the deeper waters of 


the ocean. To the westward, a rise of three thousand feet . 


would make one land of Florida, Cuba, and the Bahamas; but 
the Bahamas to the eastward would still be a line of oceanic 
islands. As to evidences of recent elevation nothing is posi- 
tively known. The great extent and height of the drift-made 
dry land appears to indicate a long resting at the present 


level. 

~The Bermuda or Somers’ Islands. —The Bermudas are 
what remains of a large atoll, as first announced by Lieutenant 
Nelson ;* and this atoll is the most remote from the equator 


of any existing. It lies in deep seas between the parallels 


1 Transactions of the Geological Society of London, 1840, v., 103. 

The following are other important publications on the structure of the Bermu- 
das: The Reports of the Challenger Expedition of 1873 and 1876, by Sir Wyville 
Thomson, London, vol. i.; ‘ The Naturalist in Bermuda,” by John Matthew Jones, 
with a map and illustrations, London, 1859; also, by the same, “ A Visitor’s Guide 
to Bermuda;” Observations on the Bermudas, in Nature for 1872; and on the geo- 
logical features of the Bermudas in the Proceedings and Transactions of the Nova 
Scotia Institute of Natural Science, 1867; A paper on the Bermuda Reefs by 
Dr. J. J. Rein, in the Senckenberg. Ber. naturforsch. Gesellschaft, 1869-70, and 
Verhandlung des I. deutsch. Geographentages fiir 1881, Berlin, 1882. There are 
also two valuable American contributions to the Geology of the Bermudas, one by 
Prof. William North Rice, of Middletown, Conn., 32 pp. 8vo, being Bulletin No. 25 
of the U. S. National Museum, 1884; and a volume by A. Heilprin, of Philadel- 
phia, entitled “ The Bermuda Islands: a contribution to the Physical History and 
Zoology of the Somers’ Archipelago,” a handsomely illustrated work of 231 pp. 8vo, 
treating of the coral reefs, and also of the zoology, and discussing at length the 
coral-island problem, with conclusions favoring, like those of Professor Rice, the 


Darwinian theory. 


THE BERMUDA ISLANDS. 219 


32° and 32° 35’, and the meridians 64° 30! and 65° 30’. The 
principal species of corals are mentioned on page 114. 

The general form and position of the reef and its islets 
are shown on the accompanying map; and its position in the 
ocean and the depths of the seas, on Plate XI. The longer 
diameter of the elliptical area trends nearly northeast-by-east, 


ed GES 
Ve 





THE BERMUDA ISLANDS. 


and is about twenty-five miles in length, while the transverse 
diameter is about fifteen miles. In the ocean about the Ber- 
mudas, the depth descends to twenty-five hundred fathoms 
and beyond. Within seventeen English miles west of the 
Bermuda reef there is a sounding (by the-Challenger Expe- 
dition) of twenty-four hundred and fifty fathoms, giving 
a slope of 1:6:1; and fifteen miles east of the southern 
submerged (in ten fathoms) reef, one of twenty-two hun- 
dred and fifty fathoms. giving the slope 1:6. There is also 
a sounding of twenty-six hundred and thirty-two fathoms 


220 CORALS AND CORAL ISLANDS. 


sixteen miles southwest of the same southern reef, giving a 
slope of 1:5°4. The soundings are too few for a decision as 
to the maximum slope. 

The emerged land, about fifteen miles long, is confined 
to the side facing southeast, excepting a single isolated rock, 
or group of rocks, on the north side (between ¢ and d on the 
map) called North Rock. It is broken into a hundred and 
fifty or more islets, and the surface is made up of hills and 
low basins. The highest point, Sears’ Hill (E), is, according 
to Lieutenant Nelson, two hundred and sixty feet in eleva- 
tion above the sea, and Gibbs Hill (D), the site of the light- 
house, two hundred and forty-five feet. Wreck Hill (F), near 
the western point of the principal island, is about one hundred 
and fifty feet high, and North Rock 1s sixteen feet high, above 
mean tide. H is the position of Hamilton, the seat of Govern- 
ment, and G, of St. George’s, the other principal town. A 
(Castle Harbor), B (Harrington Sound), and C (Great Sound) 
are three encircled bays, looking as if once the lagoons of sub- 
atolls in a Maldive-like compound atoll. The surface, about 
half way between the sounds A and B, is low. Most of the 
land is covered with cedars where not cultivated or given 
over to loose sand. The last island of the southern hook is 
Ireland Island. 

The greater part of the old atoll is still a submerged reef. 
But it is of the typical form, in having a large lagoon-like 
depression enclosed within a relatively narrow border. Its 
border is mostly from one to three fathoms under water at 
low tide, though in some parts laid bare at the ebb. It has 
open channels at a, called the Chub cut; b, Blue cut, shallow; 
ce, N. W. Channel; d, N. E. Channel ; e, Mills’ Breaker Chan- 
nel ; f, the channels affording the nearest routes to Murray 
Anchorage and St. George’s Harbor; g, Channel by St. 


THE BERMUDA ISLANDS. 221 


David’s Head, shallow; and h, Hog-fish cut. The reef- 
grounds, inside, are encumbered with countless clumps of 
corals and coral-heads, one to four fathoms under water, with 
intervals between of five to ten fathoms; some large tracts 
are without corals, and these have a nearly uniform depth of 
seven or eight fathoms. To a vessel entering, the positions 
of the coral clumps are made known by the brownish or dis- 
colored water above them. The bottom, over large areas, is 
a calcareous clay or mud; that of Murray Anchorage, a fine 
chalky clay. 

Serpule have made large accumulations over parts of the 
reef, as stated by Nelson. ‘The tubes are so heaped over one 
another as to make rings or atoll-like elevations two feet or 
so high and two to twenty feet wide. Nelson calls them 
“Serpuline reefs.’ The reefs on the east and south sides 
are narrow, not over a fourth of a mile wide, and the waters 
abruptly deepen; we may consequently conclude that this 
southeastern side of the original land was bold and _ high, 
while off to the north and west the surface was relatively 
low and flat. 

The rock of the surface is a calcareous sand-rock of wind- 
drift and beach origin like that of the Florida and Bahama 
reefs. Prof. Wm. N. Rice says that no true coral-reef rock 
is seen anywhere above the sea-level, and that the beach and 
wind-drift formations graduate into one another and are not 
easily distinguishable. The beach-made rock is in some 
places eight to fifteen feet above tide-level. Professor Rice 
says further : ' — 

“The beach-rock is, on the average, more perfectly consoli- 
dated than the drift-rock, but in this character both rocks 
vary widely. Drift-rock, when submerged by a subsidence 


} Rice, Bulletin No. 25, United States National Museum, 1884, pp. 10, 11, 14, 15. 


Doan CORALS AND CORAL ISLANDS. 


consequent to its deposition, may come to assume the degree 
of consolidation usually observed in beach-rock. On the 
south shore of the main island, near Spanish Rock, I ob- 
served strata perfectly continuous dipping toward the water, 
exceedingly hard at the margin of the water, but becoming 
considerably softer as they were traced upward and landward. 
Mr. Ebenezer Bell, who some years ago had charge of some 
works in progress on Boaz Island, informed me that he found 
that the reck, so soft as to crumble in one’s fingers, became 
quite hard on immersion for a week or a fortnight im sea- 
water. Some of the hardest rock which I observed in Ber- 
muda was shown by other characters to be unmistakably 
drift-rock. A more reliable distinction is found in the lami- 
nation, the beach-rock showing a general and uniform dip 
toward the water, while the drift-rock shows the high and 
extremely irregular dips which are characteristic of wind- 
blown sands. But not every section exhibits characters sufti- 
ciently marked to settle the nature of the rock, since the 
beach-structure admits of a considerable degree of irregu- 
larity in dip, while wind-blown sands in a long ridge or dune 
may have for long distances a gentle and nearly uniform dip. 
The indication furnished by the fossil contents of the rock is 
important. The beach-rock is often richly fossiliferous, con- 
taining shells and pieces of coral of considerable size. The 
drift-rock will, of course, ordinarily contain no relics of 
marine animals except fragments so small as to be blown 
by the wind. A high wind can, however, sweep along pieces 
of shell and coral larger than one would at first suppose. . 
“While the presence of marine fossils in a sand-rock Is 
an indication that it is a beach-rock, the drift-rock 1s quite 
apt to contain the shells of land snails. The presence of 


snail shells cannot, however, be regarded as a sure proof of 





THE BERMUDA ISLANDS. 28 


drift-rock, smmce they may easily be washed down by rains 
from a bank or bluff above the beach and imbedded in the 
beach-sands. 

“The usual softness of this drift-rock has made it a mat- 
ter of small labor and expense to secure easy grades on most 
of the roads in the islands, by making deep cuts wherever 
they are required. ‘These cuttings are of great interest to 
the geologist, from the beautiful illustration which they 
afford of that extreme irregularity of lamination which is 
characteristic of wind-drifts. Not only the country roads, 
but also the streets of the towns, abound in these beautiful 
and instructive sections. Fine exhibitions of this same struc- 
ture are to be seen in the natural sections afforded by the 
cliffs or pinnacles of the shore.” 

Professor Heilprin’s work on the Bermuda Islands con- 
tains three phototypes which show finely the general land- 
scape features of the rock. 

The great drift-sand hills, ridges, and flats, ike those of 
the Bahamas, are results of the fiercer storm-winds, as 
stated on page 155. At the Bermudas, the ordinary west- 
erly winds are feeble at transportation. But cyclones, as the 
“Sailing Directions” state, are very frequent, and “ especially 


b 


in the autumn;” and during the earlier and more furious 
half of the storm, the wind is easterly. In addition, “ Ber- 
muda squalls are sudden and violent tempests” of the winter, 
and in them the winds are from all directions. By compar- 
ing ordinary winds with violent tempests we may apprehend 
the difference at these times in the amount of force at work, 
and lose surprise over the differences in results. The “North 
Rock” is apparently the remains of drift-hills built up on the 
projecting northwest point of the reef-island ; but the surface 


must have been higher than now, for their formation. 


> 


224 CORALS AND CORAL ISLANDS. 


The Bahamas are still farther within the belt of Atlantic 
storm tracks, and in the West Indian portion, as is well shown 
on the Chart of Atlantic Storms by Wm. C. Redfield in Vol- 
ume XXXI. of the American Journal of Science, 1837. They 
are situated just outside of the continental line where the 
tracks of many of the cyclones make the turn northward ; 
and this is reason enough for high drift-heaps and the great 
width of the areas. 

The Florida region feels less powerfully the influence of 
the storms, but their influence is sufficient for the accumula- 
tion of extensive drift-ridges and a wide spread of the sands 
over the bank. 

The Bermudas have suffered greatly from erosion. There 
are no running streams, but the coral sands and the limestones 
made from them are easily dissolved and removed by carbon- 
ated waters ; and consequently the rains, reinforced in their 
carbonic acid by more from vegetable or animal decomposi- 
tion in the soil, have done a large part of the erosion over 
the surface and of that of cavern-making beneath it ; while the 
waves have made cliffs, towers, and pinnacles, and caves too, 
along the coasts. The winds, moreover, have aided both. 

Professor Rice confirms the earlier accounts of Lieutenant 
Nelson and others, and speaks of the “innumerable caves” 
as “ranging in size from miniature grottos — the bijoux 
of Nelson—to extensive caverns.” One of the miniature 
caves had been opened at Paynter’s Vale in quarrying: its 
horizontal diameter was about five feet, its height at middle 
only two; but pigmy stalagmites rose from the floor toward 
the slender stalactites that were pendent from the roof, and 
along the sides the stalactites and stalagmites were in many 
cases united to form little columns; and all was of most 


exquisite finish. 


THE BERMUDA ISLANDS. 22 


Proofs of subsidence are reported to exist in the occur- 
rence of shoreward dips of the sand-rock into and below the 
water-level (Rice); in the existence of caverns submerged 
over fifty feet, with stalagmites rismg from their floors, now 
far beneath the surface (Heilprin); and the fact mentioned 
in the Report of the Challenger Expedition, that a peat-bed, 
stumps of cedar, land snails (//elic LBermudensis), and loose 
masses of the drift-sand rock were found on Ireland Island at 
a depth below tide-level of forty feet, in the excavation for 
a floating dock. When these cedars were growing, the land 
was consequently forty-five feet or more above its present 
level. It is probable that at this time there was a long 
period of rest or cessation in the subsidence, and that during 
it the drift-sand formation of the island, including that of 
“North Rock,” was chiefly made. Afterward, when the sub- 
sidence was resumed, the work of degradation began. 

A comparatively recent elevation is indicated, according 
to Professor Rice, by the height of the beach-sand rock. fif- 
teen feet, on part of the north side of the strip of dry land. 


The origin of the “red earth” ' making much of the soil 


1 Analysis of the coral sand of Bermuda, and mean of three analyses of the 
red earth, by Mr. F. A. Manning (Agricultural Report, at Bermuda in 1873, of 
Maj.-Gen. J. H. Lefroy). 





1. Coral sand 2. Red earth. 
Warhbonicaciduer erm aces oe eS 4.06 
Ae ae gen eth tas Sich os rte oh ee OTT 5.95 
Wapnegiaie vers War Gar ist acc eeu 52a 1 109 0.36 
OLAS Estee testes cleat a ciate ae 2 BOLOG 0.14 
SIQCAMMMEE ah A Seicsccist uss. waa) esl OLD4: 0.03 
Alumina ‘ Cpe aera esate ¢ 16.94 
Tron sesquioxide ( 19.58 
SAUBNUME ACTON a 5a ees) Yee e 9 OL20 0.05 
Chlorinehysesy city ease e el ee eane ty OL02 0.015 
PhOSspHOLiclacid someway ee O08 0.70 
Sauls stt aerrpie es stele ee sre 1 OL05 56.60 
Oreanic substance’. 2) .) 2) 2) S180 15.41 
102.00 99.81 


Excluding the 15:41 per cent of organic substance in the red earth, 100 parts, 
according to the above, contain about 23 per cent of iron oxide, 20 of alumina, and 
43 of sand, besides some lime carbonate and small portions of the other ingredients 
enumerated, 


15 


226 CORALS AND CORAL ISLANDS. 


was first explained by Sir Wyville Thomson. The limestone 
contains about half of one per cent of iron oxide and earthy 
ingredients ; and these are left behind as “red earth” when 
the rest is dissolved away by the carbonated waters. 

Another source in some regions, if not at the Bermudas, 
is the volcanic dust that is widely distributed by the winds, 
or fragments of pumice and other volcanic rocks that may 
have been brought by the sea and drifting logs. Pieces of 
pumice and augitic lava have been found on the island ; and 
from the sands Mr. Murray obtained magnetite, chrysolte, 
augite. sanidin and other feldspars, mica, and perhaps quartz. 

Twenty miles southwest-by-west from the Bermudas, 
there are two submerged banks or shoals, both reported 


’ 


as having a “corally and rocky bottom ;” one has over it 
a minimum depth of twenty-four fathoms, and the other 
of ten fathoms. Dredging on these banks might make some 


interesting disclosures. 


CORALS AND CORAL ISLANDS. 227 


CHAPTER III. 


FORMATION OF CORAL REEFS AND ISLANDS, AND CAUSES OF 
THEIR FEATURES. 


I. FORMATION OF REEFS. 


I. ORIGIN OF CORAL SANDS AND THE REEF-ROCK. 


Very erroneous ideas prevail respecting the appearance of 
a bed or area of growing corals. The submerged reef’ is 
often thought of as an extended mass of coral, alive uniform- 
ly over its upper surface, and as gradually enlarging upward 
through this living growth; and such preconceived views, 
when ascertained to be erroneous by observation, have some- 
times led to skepticism with regard to the zodphytic origin 
of the reef-rock. Nothing is wider from the truth: and this 
must have been inferred from the descriptions already given. 
Another glance at the coral plantation should be taken by 
the reader, before proceeding with the explanations which 
follow. 

Coral plantation and coral field are more appropriate ap- 
pellations than coral garden, and convey a juster impression 
of the surface of a growing reef. Like a spot of wild land, 
covered in some parts, even over acres, with varied shrub- 
bery, in other parts bearing only occasional tufts of vegeta- 
tion in barren plains of sand, here a clump of saplings, and 
there a carpet of variously-colored flowers in these barren 
fields—such is the coral plantation. Numerous kinds of 
zoophytes grow scattered over the surface, like vegetation 


228 FORMATION OF CORAL REEFS AND ISLANDS. 


upon the land; there are large areas that bear nothing, and 
others of great extent that are thickly overgrown. There is, 
however, no green sward to the landscape; sand and frag- 
ments fill up the bare intervals between the flowering tufts: 
or, where the zodphytes are crowded, there are deep holes 
among the stony stems and folia. 

These fields of growing coral spread over submarine 
lands, such as the shores of islands and continents, where the 
depth is not greater than theirhabits require, just as vegetation 
extends itself through regions that are congenial. The germ 
or ovule, which, when first produced, is free, finds afterward « 


point of rock, or dead coral, or some support, to plant itself 


upon, and thence springs the tree or other forms of coral growth. 

The analogy to vegetation does not stop here. It is well 
known that the débris of the forest, decaying leaves and 
stems, and animal remains, add to the soil; that in the marsh 
or swamp—where decaying vegetation is mostly under water, 
and sphagnous mosses grow luxuriantly, ever alive and flour- 
ishing at top, while dead and dying below,—accumulations 
of such débris are ceaselessly in progress, and deep beds of 
peat are formed. Similar is the history of the coral mead. 
Accumulations of fragments and sand from the coral zo6- 
phytes growing over the reef-grounds, and of shells and other 
relics of organic life, are constantly making; and thus a bed 
of coral débris is formed and compacted. ‘There is this dif- 
ference, that a large part of the vegetable material consists of 
elements which escape as gases on decomposition, so that there 
is a great loss in bulk of the gathered mass; whereas coral is 
an enduring rock material undergoing no change except the 
mechanical one of comminution. The animal portion is but 
a mere fraction of the whole zodphyte. The coral débris and 
shells fill up the intervals between the coral patches, and the 


CORALS AND CORAL ISLANDS. 229 


cavities among the living tufts, and in this manner produce 
the reef deposit; and the bed is finally consolidated, while 
still beneath the water. 

The coral zoéphyte is especially adapted for such a mode 
of reef-making. Were the nourishment drawn from below, as 
in most plants, the solidifying coral rock would soon destroy 
all life: instead of this, the zoéphyte is gradually dying be- 
low while growing above; and the accumulations of débris 
cover only the dead portions. 

But on land, there is the decay of the year, and that of old 
age, producing vegetable debris; and storms prostrate forests. 
And there are corresponding effects among the groves of the 
sea. It has been shown that coral plantations, from which 
reefs proceed, do not grow in the “calm and still” depths of 
the ocean. They are to be found amid the waves, and usually 
extend little below a hundred feet, which is far within the 
reach of the sea’s heavier commotions. To a considerable ex- 
tent they grow in the very face of the tremendous breakers 
that strike and batter as they drive over the reefs. Here is 
an agent which is not without its effects. The enormous 
masses of uptorn rock found on many of the islands may give 
some idea of the force of the lifting wave; and there are ex- 
amples on record, to be found in various treatises on Geology, 
of still more surprising effects. 

During the more violent gales the bottom of the sea is 
said, by different authors, to be disturbed to a depth of three 
hundred, three hundred and fifty, or even five hundred feet, 
and De la Beche remarks, that when the depth is fifteen fath- 
oms, the water is very evidently discolored by the action of 
the waves on the sand and mud of the bottom. 

In an article on the Force of Waves, by Thomas Steven- 
son, of Edinburgh, published in the Transactions of the Royal 


230 CORALS AND CORAL ISLANDS. 


Society of Edinburgh (vol. xvi., 1845), it is stated as a deduc- 
tion from two hundred and sixty-seven experiments, extend- 
ing over twenty-three successive months, that the average 
force for Skerryvore, for five of the summer months, during 
the years 1843, 1844, was six hundred and eleven pounds per * 
square foot; and for six of the winter months of the same 
year, it was two thousand and eighty-six pounds per square 
foot, or three times as great as during the summer months. 
During a westerly gale, at the same place, in March, 1845, a 
pressure of six thousand and eighty-three pounds was regis- 
tered by Mr. Stevenson’s dynamometer (the name of the in- 
strument used). He mentions several remarkable instances 
of transported blocks. 

We must, therefore, allow that some effect will be pro- 
duced upon the coral groves. There will be trees prostrated 
by gales, as on land, fragments scattered, and fragmentary 
and sand accumulations commenced. Besides, masses of the 
heavier corals within ten to twenty feet of the surface may 
be uptorn, and carried along over the coral plantation, which 
will destroy and grind down every thing in their way. So 
many are the accidents of this kind to which zodphytes ap- 
pear to be exposed, that we might believe they would often be 
exterminated, were they not singularly tenacious of life, and 
ready to sprout anew on any rock where they may find quiet 
long enough to give themselves again a firm attachment. 

But it should be observed, that the sea would have far 
less effect upon the slender forms characterizing many z00- 
phytes, among which the water finds free passage, than on the 
massive rock, against whose sides a large volume may drive 
unbroken. Moreover, much the greater part of the strength 
of the ocean is exerted near tide level, where it rises in break- 
ers which plunge against the shores. Yet owing to the many 





FORMATION OF CORAL REEFS AND ISLANDS. PAS 


nooks and recesses deep among the corals, the rapidly moving 
waters, during the heavier swells, must produce whirling ed- 
dies of considerable force. tending to uproot or break the coral 
clumps. Moreover, it is to be kept in mind that shells and 
echinoderms make contributions as well as corals, and that all 
life grows luxuriantly in the coral seas. 

There is another process going on over the coral field, some- 
what analogous to vegetable decay, though still very different. 
Zodphytes have been described as ever dying while living. The 
dead portions have the surface much smoothed, or deprived 
of the roughening points which belong to the living coral, and 
the cells are sometimes half obliterated, or the delicate lamelle 
worn away. This may be viewed as one source of fine coral 
particles ; and as the process is constantly going on, it is not 
altogether unimportant. This material is in a fit condition to 
enter into solution, and it cannot be doubted that the water 
receives lime from this source, which is afterward yielded to 
the reef. 

In the Alcyonia family, which includes semi-fleshy corals, 
and in the Gorgonie, the lime is often scattered through the 
polyps in granules ; and the process of death sets these calca- 
reous grains free, which are constantly added to the coral sands. 
The same process has been supposed to take place in the more 
common reef corals, the Madrepores and Astreeas, and it is 
possible that this may be to some extent the case. Yet it 
would seein, from facts observed, that after the secretion has 
begun within the polyp, the secretion of lime going on takes 
place against the portions already formed and in direct union 
with them, and not as granules to be afterward cemented. 

The mud-like deposits about coral reefs (pp. 142, 183, 205) 
have been attributed to the causes just mentioned, but with- 
out due consideration. There is an unfailing and abundant 


232 CORALS AND CORAL ISLANDS. 


source of this kind of material in the self-triturating sands of 
the reefs acted upon by the moving waters. On the seaward 
side of coral islands, and on the shores of the larger la- 
goons, where the surface rises into waves of much magnitude, 
the finer portions are carried off, and the coarser sand remains 
alone to form the beaches. This making of coral sand and 
mud is just like that of any other kind of sand or mud. It 
takes place on all shores exposed to the waves, coral or not 
coral, and in every case the gentler the prevailing movement 
of the water, the finer the material on the shore. In the 
smaller lagoons, where the water is only rippled by the winds, 
or roughened for short intervals, the trituration is of the 
gentlest kind possible, and, moreover, the finely pulverized 
material remains as part of the shores. Thus the fine mater- 
ial of the mud must be constantly forming on all the shores, 
for the sands are perpetually wearing themselves out; but the 
particles of the fine mud, which is washed out from the beach 
sands, accumulates only in the more quiet waters some dis- 
tance outside of the reef, and within the lagoons and channels, 
where it settles. This corresponds exactly with the facts; and 
every small lake or region of quiet waters over our continent, 
illustrates the same point. 

Mr. Darwin, in discussing the origin of the finer calcareous 
mud, (op. cit., p. 14), supposes that it is derived in part from 
fishes and Holothurians; and other authors have thrown out 
the same suggestion. He cites as a fact, on the authority of 
Mr. Liesk, that certain fish browse on the living zodphytes; and 
from Mr. Allan, of Forres, he learned also that Holothurians 
subsisted on them. The statement about the Holothurians 
has been set aside by observation. Small fish swarm about 
the branching clumps, and when disturbed, seek shelter at once 
among the branches, where they are safe from pursuit. The 


FORMATION OF CORAL REEFS AND ISLANDS. 2353 


author has often witnessed this, and never saw reason to sup- 
pose that they clustered about the coral for any other purpose. 
It is an undoubted fact, as stated by Mr. Darwin, that frag- 
ments of coral and sand may be found in the stomachs of 
these animals, but this is not sufficient evidence of their 
browsing on thecoral. Fish so carefully avoid polyps of all 
kinds because of their power of stinging (as illustrated on 
p. 37), that we should wait for further and direct evidence 
on this point. The conclusion deduced by him from the facts, 
may be justly doubted. The fish and Holothurians, though 
numerous, are quite inadequate for the supply; and, more- 
over, we have, as explained above, an abundant source of the 
finest coral material without such aid. Motion of particle 
over particle will necessarily wear to dust, even though the 
particles be diamonds; and this incessant grinding action 
about reefs accounts satisfactorily for the deposits of coral 
mud, however great their extent. 

The coral world, as we thus perceive, is planted, like the 
land, with a variety of shrubs and smaller plants, and the el- 
ements and natural decay are producing gradual accumula- 
tions of material, like those of vegetation. The history of the 
growing reef has consequently its counterpart among the or- 
dinary occurrences of the land about us. 


The progress of the coral formation is like its commence- 
ment. The same causes continue, with similar results, and 
the reader might easily supply the details from the facts al- 
ready presented. The production of débris will necessarily 
continue to go on: a part will be swept by the waves, across 
the patch of reef, into the lagoon or channel beyond, while 
other portions will fill up the spaces among the corals along 
its margin, or be thrown beyond the margin and lodge on it- 


234 CORALS AND CORAL ISLANDS. 


surface. The layer of dead coral rock which makes the body 
of the reef, has its border of growing corals, and is thus un- 
dergoing extension at its margin, both through the increase in 
the corals, and the débris dropped among them. 

But besides the small fragments, larger masses will be 
thrown on the reefs by the more violent waves, and commence 
to raise them above the sea. The clinker fields of coral by 
this means produced, constitute the first step in the formation 
of dry land. Afterward, by further contributions of the | 
coarse and fine coral material, the islets are completed, and 
raised as far out of the water as the waves can reach—that is, 
about ten feet, with a tide of three feet; and sixteen to 
eighteen feet with a tide of six or seven. 

The Ocean is thus the architect, while the coral polyps af 
ford the material for the structure; and, when all is ready, it 
sows the land with seed brought from distant shores, covering 
it with verdure and flowers. 

The growth of the reefs and islands around high lands is 
the same as here described for the atoll. The reef-rock is 
mainly a result of accumulations of coral and shell débris. 
There are reefs where the corals retain the position of growth, 
as has been described on a former page. But with these the 
débris comes in to fill up the intervening spaces or cavities, and 
make a compact bed for consolidation. There are other parts, 
especially portions of the outer reef along the line of break- 
ers, which are formed by the gradual growth of layer upon 
layer of incrusting Nullipores; but such formations are of 
small extent, and only add to the results from other sources. 


II. ORIGIN OF THE SHORE PLATFORM. 


Among the peculiarities of coral islands, the shore plat- 
form appears to be one of the most singular, and its origin 


FORMATION OF CORAL REEFS AND ISLANDS. 235 


has not been rightly understood. It will be remembered that 
it lies but little above low-tide level, and it is often over three 
hundred feet in width, with a nearly flat surface throughout. 

Though apparently so peculiar, the existence of this plat- 
form is due to the simple action of the sea, and is a necessary 
result of this action. On the shores of New South Wales, 
Australia, near Sydney, as observed by the author, the same 
structure is exemplified along the sandstone shores of this 
semi-continent, where it is continued for scores of miles. At 
the base of the sandstone cliff, in most places one or more hun- 
dred feet in height, there is a layer of sandstone rock, lying, 
like the shore platform of the coral island, near low-tide level, 
and from fifty to one hundred and fifty yards in width. It is 
continuous with the bottom layer of the cliff: the rocks which 
once covered it have been removed by the sea. Its outer edge 
is the surf-line of the coast. At low-tide it is mostly a naked 
flat of rock, while at high tide it is wholly under water, and 
the sea reaches the cliff. 

New Zealand, at the Bay of Islands, affords a like fact in 
ap argillaceous sand-rock; and there was no stratification in 
this case to favor the production of a horizontal surface; it 








THE OLD HAT. 


was a direct result from the causes at work. The shore shelf 
stands about five feet above low water. A small island in this 
bay is well named the ‘Old Hat,” the platform encircling it, 
as shown in the above figure, forming a broad brim to a rude 


236 CORALS AND CORAL ISLANDS. 


conical crown. The water, in these cases, has worn away the 
cliffs, leaving a broad horizontal basement above the level of 
low tide. 

According to Professor Verrill, the same feature is exhib- 
ited on a grand scale at the island of Anticosti im the Gulf 
of St. Lawrence, and “Old Hats” are among the forms 
produced. The cliff consists of limestone, and the “ shore- 
platform” is in many places over four hundred yards wide. 

A surging wave, as it comes upon a coast, gradually rears it- 
self on the shallowing shores; finally, the waters at top, through 
their greater velocity, plunge with violence upon the barrier 
before it. The force of the ocean’s surge is therefore mostly 
confined to the summit waters, which add weight to superior ve- 
locity, and drive violently upon whatever obstacle is presented. 
The lower waters of the surge advance steadily but more 
slowly, owing to the retarding friction of the bottom; the 
motion they have is directly forward, and thus they act with 
little mechanical advantage ; moreover, they gradually swell 
over the shores, and receive, in part, the force of the wpper 
waters. ‘The wave, after breaking, sweeps up the shore till it 
gradually dies away. Degradation from this source is conse- 
quently most active where the upper or plunging portion of 
the breaker strikes. 

But, further, we observe that at low-tide the sea is compara- 
tively quiet; it is during the influx and efflux that the surges 
are heaviest. The action commences after the rise, is strongest 
from half to three-fourths tide, and then diminishes again near 
high tide. Moreover, the plunging part of the wave is raised 
considerably above the general level of the water. From 
these considerations, it is apparent that the line of greatest 
wave-action must be above low-water level. ‘ Let us suppose a 


tide of three feet, in which the action would probably be ~ 








FORMATION OF CORAL REEFS AND ISLANDS. 237 


strongest when the tide had risen two feet out of the three; 
and let the height of the advancing surge be four feet: the 
wave, at the time of striking, would stand, with its summit, 
three feet above high-tide level; and from this height would 
plunge obliquely downward against the rock or any obstacle 
before it. It is obvious that, under such circumstances, the 
greatest force would be felt not far from the line of high tide, 
or between that line and three feet above it; moreover, the 
rise of the waters to half or two-thirds tide affords a protection 
against the breaker to whatever is below this level. In re- 
gions where the tide is higher than just supposed, as six feet 
for example, the same height of wave would give nearly the 
same height to the line of wave action, as compared with high- 
tide level. Under the influence of heavier waves, such as are 
common during storms, the line of wave-action would be at a 
still higher elevation, as may be readily estimated by the 
reader. 

Besides a line of greatest wave-action, we also distinguish 
a height of feeble action, —so feeble that the rock remains 
unremoved along and below a nearly horizontal plane which 
is often three hundred to four hundred yards in width. The 
height, as is evident from the facts stated, is some distance 
above low-tide level. The lower waters of the tide, besides 
being protective, as above explained, are accumulative in their 
ordinary action, when the material exposed to them is moy- 
able; they transport shoreward, while the upper waters are 
eroding, and preparing material to be carried off. The height 
at which these two operations balance each other will be the 
height, therefore, of the line of least degradation. Moreover, 
it should vary with the height of the tides. This height, on 
the eastern shores of Australia, is three feet above ordinary 
low tide, and at New Zealand about five feet. With regard 


238 CORALS AND: CORAL ISLANDS. 


AK 
to the height varying with the tides, we observe that in the | 


Paumotus, where the water rises but two or three feet, the | 
platform is seldom over four to six inches above low tide, >~ 
which is proportionally less than at Australia and New Zea- 
land, where the tide is six and eight feet. From these ob- 
servations it appears that the height of least wave-action. 
as regards the degradation of a coast under ordinary seas, 
is situated near one-fifth tide in the Paumotus, and above 
half-tide at New Zealand, showing a great difference between 
the effect of the comparatively quiet work of the middle Pa- 
cific, and the more violent of New Zealand. Within the Bay 
of Islands, where the sea has not its full force, the platform, 
as around the ‘‘ Old Hat,” is but little above low-water level. 
The exact relation of the height of the platform to the height 
and force of the tides, and the force of wave-action, remains 
to be determined more accurately by observation. While, 
therefore, the height of the shore platform depends on the 
tides, and the degree of exposure to the waves, the breadth 
of it will be determined by the same causes in connection with 
the nature of the rock material. 

On basaltic shores it is not usual to find a shore platform, 
as the rock scarcely undergoes any degradation, except from 
the most violent seas; such coasts are consequently often cov- 
ered and protected by large fragments of the basaltic rocks. 
But on sandstone shores, if the rock is not too firm to yield 
sensibly under the stroke of the breakers, this gradual action 
keeps the platform of nearly uniform breadth. Moreover, 
any masses torn from the edge of the platform and thrown 
upon it by storm waves, or the heavier earthquake waves, 
may be soon destroyed by the same action, and carried off ; 
and thus the platform may be kept nearly clean of débris, 
even to the base of the cliff. 


} 





FORMATION OF CORAL REEFS AND ISLANDS. 239 


In the case of the coral island, the material of the coral 
platform is piled up by the advancing surges, and cemented 
through .the infiltrating waters. These surges, on reaching 
the edge of the shelf, break upon it with more or less force 

during low tide and the commencing rise; but later the 
waters swell over it before breaking, and thus throw a pro- 
tection about the exposed rocks; and as the tide continues to 
rise, they sweep over the shelf, but only clear it of sand and 
fragments, by bearing them to the beach on which they ex- 
pend their force. Where the tides are five to six feet in 
height, the shore platform of atolls is narrow. 

The isolated blocks in the Paumotus which stand on the 
platform, attached to it below, are generally most worn one 
or two feet above high-tide level, —a fact which corresponds 
with the statement in a preceding paragraph with regard to 
the height of the greatest wave-action. 


Il, EFFECTS OF WINDS AND GALES, 


In addition to this ordinary wave-action, there are also 
more violent effects from storms; and these are observed alike 
on the Australian shores referred to, and on those of coral 
islands. The waters as they move in, first draw away, and 
then drive on with increased velocity up the shallowing shores, 
or under shelving layers, and thus they easily break off great 
rocks from the edge of the platform, and throw them on the 
reef. From the observations of Mr. Stevenson, cited on a pre- 
ceding page (p. 229), it appears that the force of the waves 
during the summer and winter months differs at Skerryvore 
more than 1,200 pounds to the square foot. The seasons are 
not as unlike in the tropical] part of the Pacific. But in all 


seas there is a marked difference and in some stormy months 


240 CORALS AND CORAL ISLANDS. 


increase this difference. Further, the winds work with the 
waves, and bear the lighter part of the beach-making sands 
to a higher level than can be reached by the waves, giving 
the beach a top of wind-drift deposition as already explained. 
Still more violent in action are the great earthquake-waves, 
which move through the very depths of the ocean. 

These principles offer an explanation also of the general 
fact that the windward reef is the highest. The ordinary 
seas both on the leeward and windward sides, are sufficient 
for producing coral débris and building up the reef, and in 
this work the two sides will go on together, though at different 
rates of progress. We may often find no very great dif- 
ference in the width of the leeward and windward reefs, es- 
pecially as the wind for some parts of the year, has a course 
opposite to its usual direction. But seldom, except on the 
side to windward, is a sufficient force brought to bear upon the 
edge of the platform, to detach and uplift the larger coral 
blocks. The distance to which the waves may roll on without 
becoming too much weakened for the transportation of up- 
torn blocks, will determine the outline of the forming land. 
With proper data as to the force of the waves, the tides, and 
the soundings around, the extent of the shore platform might 
be made a subject of calculation. 

The effect of a windward reef in diminishing the force of 
the sea, is sometimes shown in the influence of one island on 
another. A striking instance of this is presented by the 
northernmost of the Gilbert Islands (see map, on page 165.) 
All the islands of this group are well wooded to windward— 
the side fronting east. But the north and northeast sides of 
Tari-tari are only a bare reef, through a distance of twenty 
miles, although the southeast reef is a continuous line of ver- 








FORMATION OF CORAL REEFS AND ISLANDS. DAW 


dure. The small island of Makin, just north of Tari-tari, is 
the breakwater which has protected the reef referred to from 
the heavier seas. 

Coral island accumulations have an advantage over all 
other shore deposits, owing to the ready agglutination of cal- 
careous grains, as explained on a following page. It has been 
stated that coral sand-rocks are forming along the beaches, 
while the reef-rock is consolidating in the water. A defence 
of rock against encroachment is thus produced, and is in con- 
tinual progress. Moreover, the structure built amid the | 
waves, will necessarily have the form and condition best fitted 
for withstanding their action. The atoll is, therefore, more 
enduring than hills of harder basaltic rocks. Reefs of 
zoophytic growth but ‘“‘mock the leaping billows,” while 
other lands of the same height gradually yield to the assaults 
of the ocean. There are cases, however, of wear from the 
sea, owing to some change of condition in the island, or in 
the currents about it, in consequence of which, parts once 
built up are again carried off. Moreover, those devastating 
earthquake-waves which overleap the whole land, may occa- 
sion unusual degradation. Yet in ordinary seas these islands 


have within themselves the source of their own repair, and 
are secure from all serious mjury. 


The change of the seasons is often apparent in the distri- 
bution of the beach sands covering the prominent points of an 
island. At Baker’s Island (near the equator, in long. 176° 
23’, W.), this fact is well illustrated. J. D. Hague states 
(Am. Jour. Scr, IL., xxxiv, 237), that the shifting sands 
change their place twice a year. ‘The western shore of the 
island trends nearly northeast and southwest; the southern 
shore, east-by-north. At their junction there is a spit of sand 


extending out toward the southwest. During the summer. 
16 


242 CORALS AND CORAL ISLANDS. 


the ocean swell, like the wind, comes from the southeast, to 
the force of which the south side of the island is exposed, 
while the western side is protected. In consequence, the sands 
of the beach that have been accumulating during the summer 
on the south side, are all washed around the southwest point 
and are heaped up on the western side, forming a plateau 
along the beach two or three hundred feet wide, nearly cover- 
ing the shore platform, and eight or ten feet deep. With 
October and November comes the winter swell from the north- 
northeast, which sweeps along the western shore, and from 
the force of which the south side is in its turn protected. 
Then the sand begins to travel from the western to the south- 
ern side; and, after a month or two, nothing remains of the 
great, sand plateau but a narrow strip; while on the south 
side, the beach has been extended two hundred or three hun- 
dred feet. This lasts until February or March, when the 


operation is repeated.” 


Il. CAUSES MODIFYING THE FORMS AND GROWTH OF REEFS. 


Coral reefs, although (1) dependent on the configuration of 
the submarine lands for many of their features, undergo vari- 
ous modifications of form, or condition, through the influence of 
extraneous causes, such as (2) wnequal exposure to the waves ; 
(8) oceanze or local currents ; (4) presence of fresh or impure 
waters. In briefly treating of these topics, we may consider 
first, reefs around high islands, and afterward, atoll reefs. 
The effect of the waves on different sides of reefs has already 
been considered, and we pass on, therefore, at once to the 
influence of oceanic or local currents, and fresh or impure 


waters. 


FORMATION OF CORAL REEFS AND ISLANDS. 245 


I, BARRIER AND FRINGING REEFS. 


The existence of harbors about coral-bound lands, and of 
entrances through reefs, is largely attributable to the action of 
tidal or local marine currents. The presence of fresh-water 
streams has some effect toward the same end, but much less 
than has been supposed. ‘These causes are recognized by 
Mr. Darwin in nearly the same manner as here: yet the 
views presented may be taken as those of an independent wit- 
ness, as they were written out before the publication of his 
work. 

There are usually strong tidal currents through the reef 
channels and openings. These currents are modified in char- 
acter by the outline of the coast, and are strongest wherever 
there are coves or bays to receive the advancing tides. The 
harbor of Apia, on the north side of Upolu, affords a striking 
illustration of this general principle. The coast at this place 





HARBOR OF APIA, UPOLU. 


has an indentation 2,000 yards wide and nearly 1,000 deep, 
as in the accompanying sketch, reduced from the chart by the 
Expedition. The reef extends from either side, or cape, a mile 
out to sea, leaving between an entrance for ships. The har- 
bor averages ten feet in depth, and at the entrance is fifteen 
feet. In this harbor there is a remarkable out-current along 
the bottom, which, during gales, is so strong at certain states 


244 CORALS AND CORAL ISLANDS. 


of the tide that a ship at anchor, although a wind may be 
blowing directly in the harbor, will often ride with a slack 
cable; and in more moderate weather the vessel may tail out 
against the wind. ‘Thus when no current but one inward is 
perceived at the surface, there is an undercurrent acting 
against the keel and bottom of the vessel, which is of sufficient 
strength to counteract the influence of the winds on the rig- 
ging and hull. The cause of such a current is obvious. The 
sea is constantly pouring water over the reefs into the harbor, 
and the tides are periodically adding to the accumulation ; 
the indented shores form a narrowing space where these waters 
tend to pile up: escape consequently takes place along the 
bottom by the harbor-entrance, this being the only means of 
exit. There are many such cases about all the islands. Ina 
group like the Feejees, where a number of the islands are 
large and the reefs very extensive, the currents are still more 
remarkable, and they change in direction with the tides, 
“Through the channels and among the inner reefs of the 
Australian reef-region,” says Jukes, ‘“‘they run sometimes 
with an impetuous sweep in the same direction even for two 
or three days together, especially after great storms have 
driven large quantities of water into the space between the 
outer edge and the land.” 

A current of the kind here represented will carry out much 
coral débris, and strew it along its course. The transported 
material will vary in amount from time to time, according to the 
force and direction of the current. It is therefore evident 
that the ground over which it runs must be wholly unfit for 
the growth of coral, since most zodphytes are readily destroy- 
ed by depositions of earth or sand, and require, for most spe- 
cies, a firm basement. Or if the flow is very strong, it will 
scour out the channels and so keep them open. The existence 


FORMATION OF CORAL RHEFS AND ISLANDS. 245 


of an opening through a reef may require, therefore, no other 
explanation ; and it is obvious that harbors may generally be 
expected to exist wherever the character of the coast is such 
as to produce currents and give a fixed direction to them. 

The currents, about the reef grounds west of the large 
Feejee Islands, aid in distributing the debris both of the land 
and the reefs. In some parts, the currents eddy and deposit 
their detritus; in others they sweep the bottom clean. Thus, 
under these varying conditions, there may be growing corals 
over the bottom in some places and not in others; and the 
reefs may be distributed in patches, when without such an 
influence we mjght expect a general continuity of coral reef 
over the whole reef-grounds. 

The results from marine currents are often increased by 
waters from the island streams; for the coves, where harbors 
are most likely to be found, are also the embouchures of val- 
leys and the streamlets they contain. The fresh waters poured 
in add to the amount of water, and increase the rapidity of 
the out-current. At Apia, Upolu, there is a stream thirty 
yards wide; and many other similar instances might be men- 
tioned. These waters from the land bring down also much 
detritus, especially during freshets, and the depositions aid 
those from marine currents in keeping the bottom clear of 
growing coral. These are the principal means by which fresh- 
water streams contribute toward determining the existence of 
harbors; for little is due to their freshening the salt waters of 
the sea. 

The small influence of the last-mentioned cause—the one 
most commonly appealed to—will be obvious, when we con- 
sider the size of the streams of the Pacific islands, and the 
fact that fresh water is lighter than salt, and therefore, in- 
stead of sinking, flows on over its surface. The deepest rivers 


DAG CORALS AND CORAL ISLANDS. 


are seldom over six feet, even at their mouths; and three or 
four feet is a more usual depth. They will have little effect, 
therefore, on the sea water beneath this depth, for they can- 
not sink below it; and corals may consequently grow even 
in front of a river’s mouth. 

Fresh-water streams, acting in all the different modes 
pointed out, are of little importance in harbor-making about 
the islands of the Pacific. The harbors, with scarcely an ex- 
ception, would have existed without them. They tend, how- 
ever, by the detritus which they deposit, to keep the bottom 
more free from growing patches of coral, and keep channels 
over the shore reef sutticiently deep and wide for a boat to 
reach the land. 

The map of the reef of North Tahiti, on page 149, and 
the following map of Matavai Bay on a larger scale, afford 
illustrations of this subject. 

a. The harbor of Papieti is enclosed by a reef about three 
fourths of a mile from the shore. The entrance through the 
reef is narrow, with a depth of eleven fathoms at centre, six 
to seven fathoms either side, and three to five close to the reef. 
This fine harbor receives an unimportant streamlet, while a 
much larger stream empties just to the east of the east cape, 
opposite which the reef is close at hand and unbroken. 

b. Toanoa is the harbor next east of Papieti. The en- 
trance is thirty-five fathoms deep at middle. and three and 
a half to five fathoms near the points of the reef. There is 
no fresh-water stream, except a trifling rivulet. 

c. Papaoa is an open expanse of water, harbor-lke in 
character, but without any entrance; the reef is unbroken. 
Yet two streams empty into it. 

d. Off Matavai, the place next east, the reef is inter- 
rupted for about two miles. The harbor is formed by an 


FORMATION OF CORAL REEFS AND ISLANDS. 247 


extension of the reef off Point Venus, the east cape. There 
is no stream on the coast opposite this interruption in the 
reef, except toward Point Venus; and at the present time 
the waters find their principal exit east of the Point, behind 


a large coral reef, a quarter of a mile distant. 


_ Scale of 500 meters 


100__200___300__400. “B00. iia 


10) & 
43° «12 


Dolphin Shoal 





PART OF NORTH SHORE OF TAHITI. 


It is evident that the growth of coral reefs is not much 
retarded about Tahiti by fresh-water streams. In fact none 
of these so-called rivers are over three feet in depth; and the 
most they can do is to produce a thin layer of brackish water 


over the sea within the channels. 


24S FORMATION OF CORAL REEFS AND ISLANDS. 


e. The following figure of the harbor of Falita, Upolu, 
represents another coral harbor, as surveyed by Lieutenant 
Emmons. At its head there is a stream twenty-five or thirty 
yards wide and three feet deep. Notwithstanding the unusual 


size of the river, the coral reef lies near its mouth, and pro- 





HARBOR OF FALIFA, 


jects some distance in front of it. Its surface is dead, but 
corals are growing upon its outer slope. 

yj. The harbor of Rewa, in the Feejees, may be again al- 
luded to. ‘The waters received by the bay amount to at least 
500,000 cubic feet a minute. Yet there is an extensive reef 
enclosing the bay, lying but three miles from the shores, and 
with only two narrow openings for ships. The case is so re- 
markable that we can hardly account for the facts without 
supposing the river’s mouth to have neared the reef by depo- 
sitions of detritus since the inner parts of the reef were formed ; 
and there is some evidence that this was the case, though to 
what distance we cannot definitely state. With this admis- 
sion, the facts may still surprise us; yet they are explained on 
the principle that fresh water does not sink in the ocean, but 
is superficial, and runs on in a distinct channel; its effect is al- 
most wholly through hydrostatic pressure, increasing the force 
of the underwater currents, and through their depositions of. 
detritus Besides these instances, there are many others in 
the Feejees, as will be observed on the chart at the end of 
this volume. Mokungai has a large harbor, without a 
stream of fresh water ;—so also Vakea and Direction Island. 


FORMATION OF CORAL REEFS AND ISLANDS. 949 


The instances brought forward are a fair example of what 
is to be found throughout coral seas; and they establish, be- 
yond dispute, that while much in harbor-making should be at- 
tributed to the transported sand or earth of marine and fresb- 
water currents, in preventing the growth of coral, but little 
is due to the freshening influence of the streams of islands. 

But while observing that currents have so decided an in- 
fluence on the condition of harbors, we should remember an- 
other prevalent cause already remarked upon, which is perhaps 
more wide in its effects than those just considered. I refer to 
the features of the supporting land, or the character of sound- 
ings off a coast. We need not repeat here the facts, showing 
that many of the interruptions of reefs have thus arisen. 
The wide break off Matavai may be of this kind. The widen- 
ing of the inner channel at Papieti, forming a space for a har- 
bor, may be another example of it; for the reef’ here extends 
to a greater distance from the shores, as if because the waters 
shallowed outward more gradually off this part of the coast. 

The same cause—the depth of soundings, on the principle that 
corals do not grow where the depth much exceeds a hundred 
feet—has more or less influence about all reefs in determining 
their configuration and the outlines of harbors. A remark- 
able instance of the latter is exemplified in the annexed chart 
of Whippey harbor, Viti Levu, reduced from the chart of the 
Wilkes Expedition to the scale of half an inch to the mile. 

The existence of harbors should therefore be attributed, to 
a great extent, to the configuration of the submarine land; 
while currents give aid in preventing the closing of channels, 
and keeping open grounds for anchorage. This subject will be 
further illustrated in the following pages. 

The permanency of coral harbors follows directly from the 
facts above presented They are secure against any immediate 


250 CORALS AND CORAL ISLANDS. 


- obstruction from reefs. Any growing patches within them 
may still grow, and the margins of the enclosing reef may 
gradually extend and contract their limits; yet only at an ex 
tremely slow rate. Notwithstanding such changes, the chan- 


nels will remain open, and large anchorage grounds clear, as 








WHIPPEY HARBOR, VITI LEVU. 

long as the currents continue in action. Coral harbors are 
therefore nearly as secure from any new obstructions as those 
of our continents. The growing of a reef in an adjoining 
part of the coast, may in some instances diminish or alter the 
currents, and thus prepare the way for more important chan- 
ges in the harbor; but such effects need seldom be feared, and 
results from them would be appreciable only after long periods, 
since, even in the most favorable circumstances, the growth 
of reefs is very slow. 

When channels have a bottom of growing coral, they form 
an exception to the above remark; for since the coral is acted 
upon by no cause sufficient to prevent its growth, the reef will 
continue to rise slowly toward the surface. 

Again, when the channels are over twenty-five fathoms 
in depth, they have an additional security beyond that from 
currents, in the fact that reef-making corals rarely grow at 
such a depth. The only possible way in which such channels 
could close, without first filling up by means of shore mate- 
rial, would be by the extension of the reefs from either side, 


FORMATION OF CORAL REEFS AND ISLANDS. 251 


till they bridge over the bottom below. But such an event 
is not likely to happen in any but narrow channels. 

In recapitulation, the existence of passages through reefs, 
and the character of the coral harbors, may be attributed to 
the following causes : 

1. The configuration and character of the submarine land ; 
—corals not growing where the depth exceeds certain limits, 
or where there is no firm rocky basement for the plantation. 

2. The direction and force of marine currents, with their 
transported detritus ;—these currents having their course 
largely modified, if not determined, as in other regions, by the 
features of the land, the form of the sea-bottom, and the posi- 
tions of the reefs, and being sometimes increased in force by 
the contributions of island streams, which add to the detritus 
and to the weight of accumulating waters. 

3. Harbors which receive fresh-water streams, or submarine 
springs of fresh-water, are more apt to be clear from sunken 
patches; and the same causes keep open shallow passages to 
the shores, where there are shore reefs. 

It should be remembered, that while the effects from fresh- 
water streams are so trifling around islands, they may be of 
very wide influence on the shores of the continents where the 
streams are large and deep, and transport much detritus. 
This point is illustrated beyond. 


UU. ATOLL REEFS, 


The remarks on the preceding pages, respecting reefs around 
other lands, apply equally to atoll reefs. There are usually 
currents flowing to leeward through the lagoon, and out, over 
or through the leeward reef, the waves with the rising tide 
dashing over the windward side, and keeping up a large sup- 


252, CORALS AND CORAL ISLANDS. 


ply, which is greatly increased in times of storms; and this ac. 
tion tends to keep open a leeward channel for the passage of 
the water. This is the common explanation of the origin of 
the channels opening into lagoons. These currents are strong- 
est when a large part of the windward reef is low, so as to 
permit the waves to break over it; andthe coral débris they 
bear along will then be greatest. When a large part of the 
leeward reef is under water, or barely at the water’s edge, the 
waters may escape over the whole, and on this account large 
reefs sometimes have no proper channels. When the land 
to windward becomes raised throughout above the sea, so as 
to form a continuous barrier which the waves cannot pass, 
the current is less perfectly sustained, since it is then dependent 
entirely upon the influx and efflux of the tides; and the leeward 
channels, in such a case, may gradually become closed. 

The action of currents on atolls is, therefore, in every way 
identical with what has been explained. The absence of 
coves of land to give force to the waters of currents, and to 
divect their course, and the absence also of fresh-water streams, 
are the only modifying causes not present. It is readily un- 
derstood, therefore, why lagoon entrances are more likely to 
become filled up by growing coral than the passages through 
barrier reefs. 

Although atolls in seas of moderate tides have the sta- 
bility stated on page 257, yet in those having tides of six 
feet or over and subject to the sweep of cyclones, they may 
find it difficult to stand their ground and repair losses above 
mean tide. But the larger reef islands may be increased in 
height by the more powerful agencies, as is well exemplified 
in the Bermuda Islands and the Bahamas. 








FORMATION OF CORAL REEFS AND ISLANDS. Oi 


Oo 


Il. RATE OF GROWTH OF REEFS. 


The formation of a reef has been shown to be a very dif- 
ferent process from the growth of a zodphyte. Its rate of 
progress is a question to be settled by a consideration of 
many distinct causes, none of which have yet been properly 
measured. 

a. The rapidity of the growth of zodphytes is an element 
in this question of great importance, and one that: should be 
determined by direct observation with respect to each of the 
species which contribute largely to reefs, both in the warmer 
and colder parts of coral-reef seas. 

b. The character of the coral plantation under consider- 
ation should be carefully studied; for it is of the greatest con- 
sequence to know whether the clusters of zodphytes are scat- 
tered tufts over a barren plain, or whether in crowded profu- 
sion. Compare the débris of vegetation on the semi-deserts of 
California with that of regions buried in foliage; equally va- 
rious may be the rate of growth of coral rock in different 
places. An allowance should also be made for the shells and 
other reef relics. The amount of reef-rock formed in a given 
time cannot exceed, in cubic feet, the aggregate of corals and 
shells added by growth—that is, if there are no additions from 
other distant or neighboring plantations. 

c. It is also necessary to examine all conditions that are 
connected with, or can influence, the marine or tidal currents 
of the region—their strength, velocity, direction, where they 
eddy, and where not, whether they flow over reefs that may 
afford débris or not. All the débris of one plantation may 
sometimes be swept away by currents to contribute to other 
patches, so that one will enlarge at the expense of others. Or, 


954 CORALS AND CORAL ISLANDS. 


a 


currents may carry the detritus into the channels or deeper 
waters around a coral patch, and leave little to aid the plan- 
tation itself in its increase and consolidation. 

d. The course and extent of fresh waters from the land, 
and their detritus, should be ascertained. 

e. The strength and height of the tides, and general force 
of the ocean waves, will have some influence. 

Owing to the action of these causes, barrier reefs enlarge 
and extend more rapidly than inner reefs. ‘The former have 
the full action of the sea to aid them, and are farther removed 
from the deleterious influences which may affect the latter. 

No results with reference to this question of the rate of 
progress in reefs were arrived at by the author in the course 
of his observations in the Pacific. The general opinion, — 
that their progress is exceedingly slow, was fully sustained. 
The facts with regard to the growth of zodphytes, give some 
data. 

Allowing that the large Madrepora of the wreck, men- 
tioned on page 126, may grow three inches in height a year, 
and other Madrepores probably three to four inches, it is still 
not easy to deduce from it the rate of increase of the reef: In 
the first place, the whole Madrepore is growing over the sides 
of its branches, at the rate, if we may judge from the size of the 
trunk at base, of a tenth of an inch a year, thus increasing 
annually the diameter a fifth of an inch a year, which, in a 
large species, is a very great addition to the three inches per 
year at the extremities of the branches. Again, the branches of 
the large Madrepore of the wreck were widely spaced, those of JZ, 
cervicornis, having intervals of from six to eighteen inches or 
more between the branches. 

In fact it is impossible to make any exact estimate of the 
amount of increase without a knowledge of the weight of the 


RATE OF GROWTH OF CORAL REEFS. 255 


part annually added. This ascertained, it would be easy to 
calculate how much the added coral would, if ground up, raise 
the area that is covered by the Madrepora. A rough esti- 
mate gives the author an average increase to this surface of 
a fourth of an inch a year. But this fourth must be much 
reduced, if we would deduce the rate of growth of the reef; 
because a large part of the reef-grounds—that is, of the region 
of soundings receiving the coral débris—is bare of growing 
corals. This is the case with much the larger portion of all 
lagoons and channels among reefs, the bottoms of which, as 
already explained, are often sandy or muddy, and to a great 
extent so because too deep for living corals; and it is true 
even of the coral plantations, these including many and large 
barren areas. These unproductive portions of reef-grounds 
constitute ordinarily at least two-thirds of the whole; and 
making this allowance, the estimate of one-fourth of an inch 
a year would become one-twelfth of an inch. 

Again, shells add considerably to the amount of calcareous 
material, perhaps one-sixth as much as the corals; but against 
this we may set off the porosity of the coral. 

The rate of growth of the Maandrina clivosa, stated on 
page 125, would make the rate of increase in the reef very 
much less rapid. ‘The specimen—the growth of fourteen 
years—weighs 24 oz. avoirdupois, and has an average diameter 
of 7 inches. ‘This gives for the amount of calcareous material 
—the specific gravity being 2°523 (p. 99)—16°45 cubic inches ; 
which is sufficient to raise a surface seven inches in diameter 
to a height of 0-428 inch; and consequently the average yearly 
increase would be about 1-33d of an inch. Allowing for two- 
thirds of the reef-ground being unproductive in corals, the 
rate of increase for the whole would become 1-100th of an 
inch. But supposing that shells add one-fourth as much as 


256 CORALS AND CORAL ISLANDS. 


the corals to the reef material, the rate of increase would be. 
come about 1-80th of an inch per year. 

The specimen of Oculina diffusa, referred to on page 125, 
weighs 44 ounces, which is five-sixths more than that of the 
Meeandrina, while the average diameter of the clump is the 
same. The average annual increase would consequently cover 
a circular area of seven inches diameter 1-18th of an inch 
deep. And making the same allowances as above, the rate 
for the year for the whole reef-grounds would be 1-44th of an 
inch. The specimen of Meandrina mentioned by Major 
Hunt, is not here made the basis of a calculation, because we 
have not the specimen for examination, and it is not certain 
that the diameter stated by him was not the horizontal 
diameter. For other facts see the Appendix. 

These estimates from the Maandrina clivosa and Oculina 
diffusa have this great source of uncertainty, that the growth 
of the groups may not have been begun in the first year of the 
fourteen. Further, the corals obtained by Major Hunt near 
Fort Taylor, Key West, may not have been as favorably situ- 
ated for growth as those of the outer margin of the reef. 
Again, we have made no allowance for the carbonate of lime 
that is supplied by the waters by way of cement, supposing 
that this must come originally, for the most part, from the 
reef itself. Besides, we have supposed, above, all the coral 
reef-rock to be solid, free from open spaces; and, further, 
it is not considered that much of it is a coral conglomerate, 
in which the fragments have their original porosity. 

On the other side, we have not allowed for loss of dé- 
bris from the reef-grounds by transportation into the deep seas 
adjoining, believing the amount to be very small. 

Whatever the uncertainties, it is evident that a reef in- 
creases its height or extent with extreme slowness. If the 








RATE OF GROWTH OF CORAL REEFS. DRG 


rate of upward progress is one-sixteenth of an inch a year, it 
would take for an addition of a single foot to its height, one 
hundred and ninety years, and for five feet a thousand years. 

It is here to be considered, that the thickness of a growing 
reef could not exceed twenty fathoms (except by the few feet 
added through beach and wind-drift accumulations), even if 
existing for hundreds of thousands of years, unless there were 
at the same time a slowly progressing subsidence; so that if 
we know the possible rate of increase in a reef, we cannot 
infer from it the actual rate for any particular reef; for it may 
have been very much slower than that. Without a subsidence 
in progress, the reef would increase only its breadth. 

In order to obtain direct observations on the rate of in- 
crease of reefs, a slab of rock was planted, by the order of Cap- 
tain Wilkes, on Point Venus, Tahiti, and by soundings, the 
depth of Dolphin shoal, below the level of this slab, was care- 
fully ascertained. By adopting this precaution, any error 
from change of level in the island was guarded against. The 
slab remains as a stationary mark for future voyagers to test 
the rate of increase of the shoal. Betore, however, the results 
can be of any general value toward determining the average 
rate of growing reefs, it is still necessary that the growing 
condition of the reef should be ascertained, the species of 
corals upon it be identified, and the influence of the currents 
investigated which sweep in that direction out of Matava 
Bay. See the map, page 247, and Appendix, page 417, 

The depth to which the shells of Tridacnas lie imbedded in 
coral rock, has been supposed to afford some data for estima. 
ting the growth of reefs. But Mr. Darwin rightly argues that 
these mollusks have the power of sinking themselves in the 
rock, as they grow, by removing the lime about them. ‘They 


occur in the dead rock,—generally where there are no growing 
i aa 


258 CORALS AND CORAL ISLANDS. 


corals, except rarely some small tufts. If they indicate any 
thing, it must be the growth of the reef-rock, and not of the 
corals themselves. But the shore-platform where they are 
found is not increasing in height; its elevation above low-tide 
being determined, as has been shown, by wave action (page 
232). They resemble, in fact, other saxicavous mollusks, sev- 
eral species of which are found in the same seas, some 
buried in the solid masses of dead coral lying on the reef. 
The bed they excavate for themselves is usually so complete 
that only an inch or two in breadth of their ponderous shells 
are exposed to’view. Without some means like this of secur- 
ing their habitations, these mollusks would be destroyed by 
the waves; a tuft of byssus, however strong, which answers 
for some small bivalves, would be an imperfect security against 
the force of the sea for shells weighing one to five hundred 


pounds. vn 


IV. ORIGIN OF THE BARRIER CONDITION OF REEFS, AND 
OF THE ATOLL FORMS OF CORAL ISLANDS. 


I. OLD VIEWS. 


In the review of causes modifying the forms of reefs, no 
reason is assigned for the most peculiar, we may say the most 
surprising, of all their features,—that they so frequently take 
a belt-like form, and enclose a wide lagoon; or, in other cases, 
range along, at a distance of some miles, it may be, from 
the land they protect, with a deep sea separating them from 
the shores. 

This peculiar character of the coral island was naturally 
the wonder of early voyagers, and the source of many specu- 
lations. The instinct of the polyp was made by some the sub- 
ject of special admiration; for the “helpless animacules” 


> 


ORIGIN OF THE BARRIER REEF 259 


were supposed to have selected the very form best calculated 
to withstand the violence of the waves, and apparently with 
direct reference to the mighty forces which were to attack the 
rising battlements. ‘They had thrown up a breastwork as a 
shelter to an extensive working ground under its lee, ‘‘ where,” 
as Flinders observes, ‘‘their infant colonies might be safely 
sent forth.” 

It has been a more popular theory that the coral struc- 
tures were built upon the summits of volcanoes ;—that the 
crater of the volcano corresponded to the lagoon, and the rim 
to the belt of land; that the entrance to the lagoon was over 
a break in the crater, a common result of an eruption. This 
view was apparently supported by the volcanic character of 
the high islands in the same seas. But since a more satisfac- 
tory explanation has been offered by Mr. Darwin, numerous 
objections to this hypothesis have become apparent, such as 
the following: } 

a. The volcanic cones must either have been subaerial and 
then have afterward sunk beneath the waters, or else they 
were submarine from the first.- In the former case the cra- 
ter would have been destroyed, with rare exceptions, during 
the subsidence; and in the latter there is reason to believe 
that a distinct crater would seldom, if ever, be formed. 

b. The hypothesis, moreover, requires that the ocean’s bed 
should have been thickly planted with craters—seventy in a 
single archipelago,—and that they should have been of nearly 
the same elevation; for if more than twenty fathoms below the 
surface, corals could not grow upon them. But no records 
warrant the supposition that such a volcanic area ever existed. 
The volcanoes of the Andes differ from one to ten thousand 
feet in altitude, and scarcely two cones throughout the world 
are as nearly of the same height as here supposed. Mount 


960 CORALS AND CORAL ISLANDS. 


Loa and Mount Kea, of Hawaii, present a remarkable instance 
of approximation, as they differ but two hundred feet ; but 
the two sides of the crater of Mount Loa differ three hundred 
and fourteen feet in height. Mount Kea, though of volcanic 
character, has no large crater at top. Hualalai, the third 
mountain of Hawaii, is 5,440 feet lower than Mount Loa. 
The volcanic summit of East Maui is 10,000 feet high, and 
contains a large crater; but the wall of the crater on one 
side is 700 feet lower than the highest point of the mountain ; 
and the bottom of the crater is 2,000 feet below the rim of 
the crater. Similar facts are presented by all volcanic regions. 

c. It further requires that there should be craters over 
fifty miles in diameter, and that twenty and thirty miles 
should be a common size. Facts give no support to such an 
assumption. 

d. It supposes that the high islands of the Pacific, in the 
vicinity of the coral islands, abound in craters; while, on the 
contrary, there are none, so far as is known, in the Marquesas, 
Gambier, or Society Group, the three which lie nearest to 
the Paumotus. Even this supposition fails, therefore, of giv- 
ing plausibility to the crater hypothesis. 

Thus at variance with tacts, the theory has lost favor, and 
it is now seldom urged. 

The question still recurs with regard to the basement 
of coral islands, and the origin of their lagoon character. 


ORIGIN OF BARRIER REEFS AND ATOLLS. 261 


DARWIN’S THEORY OF THE ORIGIN OF BARRIER REEFS 
AND ATOLLS. 


Mr. Darwin, in his voyage around the world as natu- 
ralist of the expedition of the “Beagle,” under Captain 
Fitzroy, R. N., during the years 1832 to 1836, visited and 
investigated the Keeling atoll in the Indian Ocean, and the 
barrier and fringing reefs of Tahiti. With the facts thus 
gathered, he had a key to all descriptions and maps of the 
reefs and reef islands of the oceans, and through careful 
study of the resources at hand, he arrived at a comprehen- 
sive knowledge of the facts and a theory of their origin.' 
The voyage of the author in 1858 to 1842 brought the sub- 
ject to his attention, and afforded him abundant illustrations 
of all sides of the subject and elucidations of some points 
which had been deemed obscure; and he believes that the 
collected facts place the theory on a firm basis of evidence. 

Darwin's theory is this: that a fringing reef skirting 
an ordinary island becomes changed by means of a slow 
subsidence and the compensating upward growth of the 
corals into a barrier reef; and that the barrier reef, by the 
continuation of the sinking until the old island has dis- 
appeared, and by the same process of growth, becomes finally 
an atoll. 


1 The third edition of Darwin’s work, issued in 1889, contains a valuable appen- 
dix by Prof. T. G. Bonney, giving a full review of the new contributions to the sub- 
ject of coral reefs, and his own views confirmatory of those of Darwin. 

® The author, besides working among the reefs of Tahiti, the Samoan (or Navi- 
gator) Islands, and the Feejees (at this last group staying three months), was also 
twice at the Hawaiian Islands. In addition, he landed on and gathered facts from 
fifteen coral islands, — seven of these in the Paumotu Archipelago; one, Tongatabu, 
in the Friendly Group; two, Taputeuea and Apia, in the Gilbert Group; and five 
others near the equator, east of the Gilbert Group, Swain’s, Fakaafo, Oatafu (Duke 
of York’s), Hull, and Enderbury’s Island. 


262 CORALS AND CORAL ISLANDS. 


In sustaining the theory, the fact of the subsidence re- 
quires proof, and secondly, its sufficiency for the result 
claimed. 

Darwin gives as evidence of the subsidence the near 
identity of barrier-girt islands and atolls. He compares 
the two, points out the fact that a slight change in the 
former by submergence is all that is required to convert 
it into an atoll, and enforces the argument by pointing to 
transitions between the two states. 

The facts from the Feejee Archipelago illustrate the sub- 
ject well. On the map, Plate XII., let the reader glance 
successively at the islands Goro, Angau, Nairai, Lakemba, 
Argo Reef, Exploring Isles, and Nanuku. It will be ob- 
served that in Goro the reef closely encircles the land upon 
whose submarine shores it was built up. In the island next 
mentioned, the reef has the same character, but 1s more dis- 
tant from the shores, forming what has been termed a bar- 
rier reef; the name implying a difference in position, but 
none in mode of formation. In the last of the islands 
enumerated, the barrier reef includes a large sea, and the 
island it encloses is but a rocky peak within this sea. 

If, now, the island Angau were sinking slowly, at a rate 
not more rapid than that of the upward growth of the reef, 
there would be a gradual disappearance of the land beneath 
the waters, while the reef might keep its level unchanged. 
Should the sinking go on until the land had mostly gone, 
the condition would be like that of the Exploring Isles, in 
which only a single ridge and a few isolated summits stand 
above the waters; and, at a stage beyond when only a single 
peak was left, the reef-girt island would have become a 
Nanuku. The subsidence of Goro, on the same principle, 


would produce an Angau. 











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ORIGIN OF BARRIER REEFS AND ATOLLS. 263 


The steps in the process are illustrated in the following 
sections of an island and its reefs. If the water-level be at 
I, the island enclosed would be, like Goro, one with a simple, 
fringing reef f, f Suppose a submergence to have gone on 
until II is the water line: —the reef growing upward may 
then have the surface represented by 0’ f’, b’ f’. There is 
here a fringing reef (/’), and also a barrier reef (6’), with a 
narrow channel (c’) between, such as exists on the shores of 
Tahiti (p. 149). Suppose a further submergence, till III is 
the water line: then the channel (c” ¢”), within the barrier 


has become quite broad, as in the island of Nairai or Angau; 





SECTION ILLUSTRATING THE ORIGIN OF BARRIER REEFS. 


on one side (f’”’) the fringing reef remains, but on the other 
it has disappeared, owing, perhaps, to some change of cir- 
cumstance as regards currents which retarded its growth 
and prevented its keeping pace with the subsidence. With 
the water at IV, there are two islets of rock in a wide lagoon, 
along with other islets (¢” 2”) of reef over two peaks which 
have disappeared. 6” b’” are sections of the distant enclos- 
ing barrier, and c” c”, and other intermediate spots, of the 
water within. The coral reef-rock by gradual growth has 
attained a great thickness, and envelops nearly the whole of 
the former land. Nanuku, the Argo Reef, and Exploring 
Isles are here exemplified ; for the view is a good transverse 
section of either of them. 


The supposed similarity between these ideal sections and 


264 CORALS AND CORAL ISLANDS. 


existing islands is fully sustained by actual comparison. The 
figure beyond is a map of the island of Aiva, in the Feejee 
Group. There are two peaks in the lagoon, precisely as 
above; and although we have no soundings of the waters in 
and about it, nor sketches of peaks, facts observed elsewhere 
authorize in every essential point the transverse section here 
viven, which resembles closely, as is apparent, the preceding. 
The section is made through the line b db, b’ b’, of the map. 


It is unnecessary to add other illustrations. They may be made 











MAP AND IDEAL SECTION OF AIVA ISLAND. 


out from any of the eastern groups of the Feejees, the Gamb- 
ier Group of the Paumotus, or Hogoleu in the Carolines. 

It has been urged against the theory, that the process ap- 
pealed to ought to fill the channels inside as the island sinks, 
and thus a plane of coral result, instead of an outside barrier 
reef and narrow belts within. 

But the facts prove that the existence of ner passages 
is a necessary feature of such islands. It has been shown 
that the ocean acts an important part in reef-making, that 
the outer reefs, exposed to its action and to its pure waters, 
grow more rapidly than those within, which are under the in- 
fluence of marine and fresh-water currents and transported 
detritus. It is obvious, therefore, that the former may retain 


themselves at the surface, when through a too rapid subsi- 


ORIGIN OF BARRIER REEFS AND ATOLLS. 26d 


dence the inner patches would disappear. Moreover, after 
the barrier is once begun, it has growing corals on both its 
inner and outer margins, while a fringing reef grows only on 
one margin. Again, the detritus of the outer reefs is, to a 
great extent, thrown back upon itself by the sea without and 
the currents within, while the inner reefs contribute a large 
proportion of their material to the wide channels between 
them. These channels, it is true, are filled in part from the 
outer reefs, but proportionally less from them than from the 
inner. The extent of reef-grounds within a barrier, raised 
by accumulations at the same time with the reefs, is often 
fifty times greater than the area of the barrier itself. Owing 
to these causes, the rate of growth of the barrier may be at 
least twice more rapid than that of the inner reefs. If the 
barrier increases one foot in height in a century, the inner 
reef, according to this supposition, would increase but half a 
foot ; and any rate of subsidence between the two mentioned, 
would sink the inner reefs more rapidly than they could grow, 
and cause them to disappear. There is therefore no objec- 
tion to the theory from the existence of wide channels and 
open seas; on the contrary, they afford an argument in its 
favor. 

From these remarks on the channels and seas within 
barrier reefs, we pass to an illustration of the origin of an 
atoll. The inference has probably been already made by the 
reader, that the same subsidence which has produced the dis- 
tant barrier, if continued a step farther, would produce the 
lagoon island. Nanuku is actually a lagoon island, with a 
single mountain peak still visible; and Nuku Levu, north of 
it, is a lagoon island, with the last peak submerged. The 
Gambier group, near the Paumotus, appears to have afforded 


an early hint with regard to the origin of the atoll, or at 


266 CORALS AND CORAL ISLANDS. 


least the close relations of the two. Captain Beechey, in his 
“ Voyage in the Pacific,” implies this resemblance, when he 
says of the Gambier group, which he surveyed, “It consists 


of five large islands and several small ones, all situated in a 





GAMBIER ISLANDS. 


lagoon, formed by a reef of coral.” Balbi, the geographer, as 
Mr. Darwin remarks, describes those barrier reefs which 
encircle islands of moderate size, by calling them atolls with 
high lands rismg from their central expanse. 

The manner in which a further subsidence results in 
producing the atoll is illustrated in the following figures. 
Viewing V as the water line, the land is entirely sub- 
merged ; the barrier (b’” b””) then encloses a broad area of 
waters, or a lagoon, with a few island patches of reef over 
the peaks of the mountains. A continuation of the subsi- 
dence would probably sink beneath the waters some of the 
islets, because of their increasing in height less rapidly than 
the barrier; and this condition is represented along the upper 
line of the above Figure VI, subsidence having taken place 
to that level. The lagoon has all the characters of those of 
atoll reefs. 


Should subsidence now diminish greatly or cease, the 


ORIGIN OF BARRIER REEFS AND ATOLLS. 267 


reefs, no longer increasing in height, would go on to widen, 
and the accumulations produced by the sea would commence 
the formation of dry land, as exhibited in figure 2. Verdure 
may soon after appear, and the coral island will finally be 
completed. 

All the features of atolls harmonize completely with this 
view of their origin. In form they are as various and irreg- 
ular as the outlines of barrier reefs. Compare Angau of the 
Feejees, with Tari-tari of the Gilbert Group (p. 165); Nairai 


or Moala with Tarawa; Nanuku with Maiana or Apamama. 





SECTION ILLUSTRATING THE ORIGIN OF ATOLLS. 


The resemblance is close. In the same manner we find the 
many forms of lagoon reefs represented among barrier reefs. 
We observe, also, that the configurations are such as would 
be derived from land of various shapes of outline, whether a 
narrow mountain ridge (as in Taputeuea, one of the Gilbert 
Islands), or wide areas of irregular slopes and mountain 
ranges. Among the groups of high islands, we observe that 
abrupt shores may occasion the absence of a reef on one side, 
as on Moala; and a like interruption is found among coral 
islands. Many of the passages through the reefs may be thus 


accounted for. 


268 CORALS AND CORAL ISLANDS. 


The fact that the submerged reef is often much prolonged 
from the capes or points of a coral island, accords well with 
these views. These points or capes correspond to points in 
the original land, and often to the lme of the prominent 
ridge; and it is well known that such ridge limes often ex- 
tend a long distance to sea, with slight inclination compared 
with the slopes or declivities bounding the ridge on either 
side. 

The derivation of the forms of reef islands from a former 
mountain range is further sustained, according to Darwin, by 
the occurrence of coral islands or reefs in chains, like the peaks 





20 of an ind to a mile. 


MENCHICOFF ATOLL. 


of such a range. He gives as an example Menchicoff atoll, of 
the Caroline Archipelago, which consists of three long loops 
or lagoon islands, united by their extremities, and which fur- 
ther subsidence might reduce to three islands. 

Darwin, in his account of the Maldives, points out indica- 
tions of a breaking up of a large atoll into several smaller 
atolls. The land with many summits or ranges of heights 
may at first have had its single enclosing reef; but as it sub- 
sided, this reef, contracting upon itself, may have encircled 
separately the several ranges of which the island consisted. 


ORIGIN OF BARRIER REEFS AND ATOLLS. 269 


and thus several atoll reefs may have resulted in place of the 
large one; and, further, each peak may have finally become 
the basis of a separate lagoon island, under a certain rate 
of subsidence or variations in it, provided the outer reef were 
so broken as to admit the influence of waves and winds. 
Some of the large atolls of the Maldives are properly atoll 
archipelagoes. 

The sizes of atolls offer no objection to these views, as 
they do not exceed those of many barrier reefs. 

All the conditions from fringing to barrier and from the 
barrier island to the atoll are admirably illustrated in the 
Louisiade Archipelago, Plate VII. The small amount of 
included high land within the enormous barrier, the linear 
form of the high islands, and the many islets which continue 
the line westward, the appendage-like relation to the large 
barrier-island of the islands at its northeast and northwest 
ends, look as if all the pieces of high land were parts of a 
nine-tenths-buried mountain-chain; and so much like it that 
any other supposition is evidently unreasonable. 

According to the principles explained and the facts illus- 
trating them, an atoll that 1s wooded through the larger part 
of its circuit, especially if not below medium size, bears evi- 
dence in this fact that the subsidence through which it was 
formed has probably ceased ; and on the contrary that atolls 
which are wholly or mostly covered with the sea at high tide, 
with few islets above high-water mark, are still undergoing 
subsidence. On this principle we may infer that the larger 
part of the Paumotus have passed to a period of cessation of 
subsidence, and that Keeling atoll in the Indian Ocean is of 
like character. Many of the northern Carolines, on the con- 
trary, may be still subsiding. 

It is of interest to follow still further the subsidence of a 


270 CORALS AND CORAL ISLANDS. 


coral island, the earlier steps in which are illustrated in the 
preceding figures. 

It is to be noted in this connection that if an atoll-reef is 
not undergoing subsidence the coral and shell material pro- 
duced that is not lost by currents serves: (1) to widen the 
reef; (2) to steepen, as a consequence of the widening, the 
upper part of the submarine slopes; (3) to accumulate, on 
the reef, material for beaches and dry land; and (4) to fill 
the lagoon. In regions of barrier reefs the inner channel 
may be a large receiver, like the lagoon of the atoll. But if, 
while subsidence is in progress, the contributions from corals 
and shells exceed not greatly or feebly the loss by subsidence 
and current waste, the atoll-reef, unable to supply sufficient 
debris to raise the reef above tide-level by making beaches 
and dry-land accumulations, would (1) remain mostly a bare 
tide-washed reef; (2) lose in diameter or size hecause the 
debris that is not used to keep the reef at tide-level is carried 
over the narrow reef to the lagoon by the waves whose throw 
on all sides is shoreward; (5) lose in irregularity of outline 
and thus approximate toward an annular form; (4) lose the 
channels through the reef into the lagoon by the growth of 
corals and by consolidating debris; and (5) become at last a 
small bank of reef-rock with a half obliterated lagoon-basin. 
Some of the islands of the equatorial Pacific in this last con- 
dition are described on page 198. | 

Moreover, the subsidence, if more rapid than the increase 
of the coral reef, would become fatal to the atoll, by gradu- 
ally sinking it beneath the sea. Such a fate, as stated by 
Darwin, has actually befallen several atoll-formed reefs of 
the Chagos Group, in the Indian Ocean (p. 192); one of 
them has only “two or three very small pieces of living reef 
rising to the surface.’ Darwin calls such reefs dead reefs. 





ORIGIN OF BARRIER REEFS AND ATOLLS. yy a | 


=i 


The southern Maldives have deeper lagoons than the northern, 
fifty or sixty fathoms being found in them. This fact indi- 
cates that subsidence was probably most extensive to the 
south, and perhaps also most rapid. The sinking of the 
Chagos Bank, which lies farther to the south in nearly 
the same line, may therefore have had some connection with 
the subsidence of the Maldives. Other drowned atoll reefs, 
of similar character, exist in the China Sea and to the north 
of Madagascar. 

The submerged Macclesfield Bank, and the Tizard Bank 
five degrees farther south, have been described by W. J. L. 
Wharton! and Captain Aldrich? The Tizard Bank is 10 miles 
broad, has depths of 30 to 47 fathoms in the lagoon part, and 
4 to 10 fathoms over the border on which alone are growing 
corals; but the border extends to the surface in eight places 
and at three of them are islets. The Macclesfield Bank is 
70 by 40 miles in area; it is like the Tizard, but lies deeper, 
the lagoon in places being 40 to 60 fathoms under water and 
the margin 4 to 10 fathoms. The Saya de Malha Bank, east 
of northern Madagascar, measures two degrees across, has 
depths of 60 to 70 fathoms within the lagoon part, and a 
border on the north and northeast sides at a depth of 10 to 
17 fathoms, a portion of which comes within 8 to 9 fathoms 
of the surface. Darwin remarks on its close resemblance to 
reefs of the Chagos Bank. To the south is the submerged 
Nazareth Bank, and Cargados Carajos at the south end of 
a reef region common to the two; and to the northeast, the 
Seychelles, of great area. The Seychelles have some granite 
islands near the centre, but constitute otherwise a great sub- 
merged bank; on the western border are shoals 3 to 7 fath- 


1 Nature, 1888, Feb. 23. 
2 Bulletin of Hydrographic Department, London, for February, 1889. 


Pie CORALS AND CORAL ISLANDS. 


oms under water more or less covered with growing corals. 
The seas about these sunken reefs of the Indian Ocean are 
2,000 to 2,600 fathoms in depth. Nearer to northern Mada- 
gascar there are coral islands that are not submerged. 

The following are other evidences in favor of the theory 
of subsidence. 

The theory explains all the varying depths of lagoons, 
from the condition of near obliteration to that of a basin 
one to three hundred feet deep. 

It gives a satisfactory reason for the existence of great 
and abrupt depths about many atolls, and off great barriers, 
and the steepest of submarine declivities. The powers of 
growth in the reef, through the limestone material derived 
from the waters by the polyps, enable it to keep itself at the 
water-level in spite of the deepening that is going on. Such 
facts as those from the depths alongside of the eastern Baha- 
mas, the reef-islands southwest of Cuba, and many atolls in 
the deeper oceans have here their full elucidation. 

The sunken atolls of the ocean, like the Chagos Bank 
(page 191), derive from Darwin’s theory their only explana- 
tion. The basin-shaped reef with high borders, the bottom 
dead because of the too great depth, the borders in places 
growing corals and having some surface islets over spots 
of more luxuriant growth where rate of progress was suffi- 
cient to keep up with the subsidence, —all the facts are a 
natural consequence of the method of origin. A model of 
such a lagoon-bank with its raised margin and few tall and 
steep islet towers, 20 to 30 feet above the rest of the border, 
would be one of the best of demonstrations for the subsidence 
theory. The coral-growing areas over the great lagoons of 
atolls and the barrier-bounded channels of the Feejees and 
other archipelagoes and those of the outer waters about 


ORIGIN OF BARRIER REEFS AND ATOLLS. 273 


islands or their barriers, show no tendency to grow with 
large depressed centres, but rather with flat. tops, as vegeta- 
tion might grow, or else with elevated centres. It is only 
when nearing the surface where the waves can help vigor- 
ously the growth and the accumulation of material on the 
border and injure the interior corals, that anything like 
a lagoon-basin begins and the atoll takes shape; and it is 
only through continued subsidence under such conditions 
that the margin can be made to grow so much faster than | 
the interior as to produce thereby a basin-like interior 50 to 
300 feet deep. Corals will grow most rapidly where food is 
most freely supplied by currents, as observed by Mr. Agassiz. 
The principle serves to explain the unequal progress in some 


reef-banks; but food is seldom deficient. Darwin draws a 
good argument for his theory, also, from the fact that lines 


of coral islands are the continuations of lines of high islands, 
and, also, that lines of the two kinds of islands often run 
parallel with one another; for example, the Hawaiian 
line of high islands four hundred miles long, ending off to 
the westward in a longer line of atolls, and the many 
parallel lines in the Pacific. | 
There is, further, not merely probable but positive evi- 
dence of subsidence in the deep coast-indentations of the 
high islands within the great barriers. The long points and 
deep fiord-like bays are such as exist only where a land, after 
having been deeply gouged by erosion, has become half sub- 
merged. The author was led to appreciate this evidence 
when on the ascent of Mt. Aorai on Tahiti, in September of 
1839." Sunk to any level above that of five hundred feet, 


1 A map of Tahiti and an account of the ascent are contained in the author’s 
“Volcanoes and Hawaiian Volcanic Phenomena.” ‘The map is published, also, in 
the American Journal of Science, 1886, xxxii, 247. 

18 


274 CORALS AND. CORAL ISLANDS. 


the erosion-made valleys of Tahiti would become deep bays, 
and above that of one thousand feet, fiord-like bays, with the 
ridges spreading in the water like spider’s legs; and this is a 
common feature of the islands and islets‘within the lagoons 
of barrier islands. The evidence of subsidence admits of no 
doubt. It makes the conclusion from the Gambier group 
positive ; and equally so that for Raiatea and Bolabola repre- 
and that 
for the Exploring Isles and others of the Feejee group; and 


> 


sented on the charts in Darwin’s “Coral Islands ;’ 
? 


that for islands, great and small, in the Louisiade Archipelago 
and in other similar groups over the oceans. 

Other arguments for the thickening of reefs through sub- 
sidence are afforded by the existence of elevated reefs, and of 
sunken and buried fringing reefs. 

The island of Metia is 250 feet in height (p. 195), full 
twice the coral-growing depth, and consists of horizontally 
stratified limestone. At the island of Mangaia, in the Her- 
vey group, the coral rock is raised 300 feet out of water. 
Such thick beds could not have been made by corals growing 
in depths not exceeding 150 feet without a sinking of scores 
of feet during their progress. 

Christmas Island, in the eastern part of the Indian Ocean, 
according to Mr. J. J. Lister and Captain Aldrich, R. N., al- 
though 1,200 feet high, has a series of horizontal terraces of 
coral-made rock to the top. There is at bottom a vertical 
cliff of 30 feet; then above the 120-foot level, a cliff of 85 
feet begins; above that of 475 feet, two cliffs together of 95 
feet; next a steep, rough slope for 650 feet of the height, 
ending in a top layer. Captain Aldrich, who ascended to the 
summit, says that he saw no rock but coral rock, and implies 
that the rock is in successive layers. The facts, taken as 
stated, prove a thickness of reef-rock of 1,200 feet. One 


ORIGIN OF BARRIER REEFS AND ATOLLS. ahh 


writer has since said, that perhaps the coral rock encases a 
voleanic mountain ; another has gone further and dropped the 
perhaps. But these statements are at present unwarranted. 
On Cuba, according to Prof. W. O. Crosby,’ coral rock 
occurs in successive terraces up to a height of nearly 2,000 
feet, and, excepting small breaks, makes the circuit of the 
island. The terraces are described as a striking feature in 
the view from the water. One terrace-plain, at 30 feet ele- 
vation, is in places nearly a mile wide and extends almost 
horizontally for hundreds of miles. <A second, of 200 to 250 
feet, rises steeply from the imner edge of the first, and a , 


third reaches a level of 500 feet. Near Havana the eleva“ 


tion is over 1,200 feet, and the rock, as reported by Mr. A. 
Agassiz, is true reef-rock. The mountain El Yunque, five 
miles west of Baracoa, 1,800 feet in height, is_volcanic rock 
below, but coral for the upper 1,000 feet. Further coral lime- 
stone has been found by Mr. Sawkins, on J amaica, at a height 
of 2,000 feet. “My. Crosby concludes that the great thickness 
of the now elevated reefs could have been produced only 
“ during a progressing subsidence,’ so that “we have appar- 
ently no recourse but to accept Darwin’s theory.” 


We are thus led to the conclusion that each coral atoll ~~ ‘\ 


once formed a fringing reef around a high island. The 
fringing reef, as the island subsided, became a barrier reef, 
which continued its growth while the land was slowly dis- 
appearing. The area of waters within finally contained the 
last sinking peak. Another period, and this had gone, leay- 
ing only the barrier at the surface and an islet or two of 
coral in the enclosed lagoon. Thus the coral wreath en- 
circling the lofty island becomes afterward its monument, 
and a record of its past existence. The Paumotu Archi- 


1 Proceedings of the Boston Society of Natural History, 1882-1883, xxii. 124. 


276 CORALS AND CORAL ISLANDS. 


pelago is accordingly a vast island cemetery, where each 
atoll marks the site of a buried island; and the whole Pacific 
is scattered over with these simple memorials. 

In view of the facts that have been presented, it is fur- 
ther evident that a barrier reef indicates approximately the 
former limits of the land enclosed. The Exploring Isles 
(Feejee chart), instead of having an area of only siz square 
miles. the whole extent of the existing land, once covered 
three hundred square miles; and the outline of the former 


land is indicated by the course of the enclosing reef. A still 


greater extent may be justly inferred. For since a barrier, 


as subsidence goes on, gradually contracts its area, owing to 
the fact that the sea bears a great part of the material 
inward over the reefs, the declivity forming the outer limit 
of the sub-marine coral formation has a steep angle of in- 
clination. In the same manner it follows that the island 
Nanuku, instead of one square mile, extended once over two 
hundred square miles, or had two hundred times the present 
area of high land. Bacon’s Isles once formed a large trian- 
gular island of equal extent, though now but two points of 
rock remain above the water. 

The two large islands in the western part of the group 
Vanua Levu and Viti Levu, have distant barriers on the 
western side. Off the north point of the former island, the 
reef begins to diverge from the coast. and stretches off from 
the shores till it is twenty and twenty-five miles distant ; 
then, after a narrow interruption, without soundings, the 
Asaua islands commence in the same line, and sweep around 
to the reef which unites with the south side of Viti Levu; 
and, tracing the reef along the south and east shores, we find 
it at last nearly connecting with a reef extending southward 
from Vanua Levu. Thus these two large islands are nearly 





OBJECTIONS TO THE THEORY OF SUBSIDENCE. Oy, 


encircled in a single belt; and it would be dog no violence 
to principles or probabilities to suppose them once to have 
formed a single island, which subsidence has separated by 
inundating the low intermediate area. We may thus not 
only trace out the general form of the land which once occu- 
pied this large area (at least 10,000 square miles), but may 
detect some of its prominent capes, as in Wakaia and Direc- 
tion Island. The present area is not far from 4,500 square 
miles. The Feejee Group, exclusive of coral islets, includes 
an area of about 5,500 square miles of dry land; while, at 
the period when the corals commenced to grow, there were 
at least, as the facts show, 15,000 square miles of land, or 
nearly three times the present extent of habitable surface. 


» | 
HAA A 
oe 


oT ee 


“OBJECTIONS TO THE SUBSIDENCE THEORY. 


The objections to the theory of coral reefs which have 
been recently urged are mostly independent hypotheses which 
are supposed to meet the facts without requiring subsidence, 
and not strictly objections to Darwin’s theory. Two or three 
real objections, however, are among them. The discussion 
brings out many points of interest. 

An improbability. — So extremely slow a subsidence, keep- 
ing pace so well with the upward growth, is improbable. 
This objection is put forth by those who are not aware that 
so slow subsidences are those with which geology is most 
familiar. A movement of the kind has been proved to be 
in progress along the coast of New Jersey and some other 
parts of the North American Atlantic border, and in western 
Greenland ; and geology is now inquiring as to whether any 
regions are absolutely stable, or wholly free from movement 
up or down. 


278 CORALS AND CORAL ISLANDS. 


Another improbability. — So great a subsidence is declared 
improbable. The thickness of coral limestone, attributed un- 
der the theory to some coral-reef formations, is pronounced 
by Dr. J. J. Rein* to be far beyond that of any reefs of earlier 
time, and therefore improbable. But precedent, whether a 
fact or not, settles nothing. Coral-made limestones are not 
essentially different in origin from shell-made limestones ; 
and the past year we have reported from Mexico, by Dr. C. A. 
White, a Cretaceous limestone, having similar fossils from 
bottom to top, and yet 4,000 feet thick. It could not have 
been made, as Dr. White states, without 4,000 feet of slowly 
progressing subsidence, for the fossils prove that it is not of 
deep-water origin. Nevada has Devonian limestones, accord- 
ing to Hague and Walcott, 6,000 feet thick, and some simi- 
lar fossils in the upper and lower portions which are proof 
of a gradual subsidence. No thick formation of any kind of 
rock was ever made, or could be made, by shore or shallow- 
sea operations without a slowly continued subsidence or a 
corresponding change of water level. 

But there is another improbability and it is the most im- 
probable of all suppositions that can be made with regard 
to the globe; that, while the continents have had their great 
changes of level, both upward and downward by the thou- 
sands of feet, the oceanic basins, of nearly three times the 
area, and volcanic in the constitution of many high islands, 
have had no changes of level except some local elevations, 
for this is implied by the objectors; not a subsidence any- 
where of a thousand feet or even a hundred, in the breadth 
of thirty millions of feet between America and the East India 
Seas. Whence this stability ? 


1 Dr. Rein’s Memoir on Bermuda is mentioned in the note on page 218. The 
above argument is given here from the citation by Dr. Geikie, the publication not 
being accessible to the writer. . 


OBJECTIONS TO THE THEORY OF SUBSIDENCE. 279 


The Elevation Theory. — The view has been presented 
that, in place of subsidence, elevation is at the bottom in 
the origin of barriers and atolls. Coral reefs may, like sea- 
beaches, be made at different heights on the slopes of rising 
land; but this is not the result of elevation which is implied ; 
for barrier reefs and atolls are the objects whose origin is to 
be accounted for. 

a. Mr. G. C. Bourne’ found at Diego Garcia, an atoll of 
the Indian Ocean southeast of the Chagos Bank, that in 
wells sunk over the island, the rock, for a short way down, 
consists of horizontal layers, eighteen inches to three feet 
thick, of coral sand in two or three alternations with coral 
shingle containing corals in coarse masses; and he concludes 
that the coarser layer corresponds to a layer of growing corals 
and the sand layer to a deposit of sand that was spread over 
and killed the corals; and that this process was repeated two 
or three times or more. Then he says: “ aise the forma- 
tion to the surface and you get that stratification which you 
see on so many parts of the island, a stratification which can- 
not be explained on any theory of subsidence.” He does not 
seem to be aware that the elevation did not make the strati- 
fication; and that the stratification is on his view positive 
evidence of a period of submergence; and that the thicken- 
ing of such a series by additions above, as in the process 
described, would require a continued subsidence previous to 
the final elevation. In any case, the final emergence is all 
there is in such facts to support an elevation theory of coral 
islands. 

The case of Christmas Island, 1,200 feet high (p. 274), 
is one of upheaval; but the upheaval, if the horizontal ter- 
races are parts of successive layers, did not make the layers. 


1 Bourne, Nature, April 5, 1888. 


280 CORALS AND CORAL ISLANDS. 


If they are only fringing reefs marking different stages in 
the elevation, they are like elevated sea-beaches, and if each 
is no thicker than the depth of growing corals, they have no 
bearing on any theory of coral islands. But the descriptions 
say that only coral rock exists on the island. 

b. The fact that elevated reefs and other evidences of 
elevation occur at the Pelews, a region of wide barrier reefs 
and atolls, has been presented by Prof. Karl Semper,’ after a 
study of those islands, as bearing in the same direction; that 
is, against the theory of subsidence, for by the subsidence 
theory we have in such facts (in the words of another), “a 
cumbrous and entirely hypothetical series of upward and 
downward movements.” Professor Semper reports the exist- 
ence of reefs raised 200 to 250 feet above the sea-level in the 
southern third of the larger of the islands, while the other 
two thirds exhibit evidence of but little, 1f any, elevation. 
The facts are like those of other elevated reefs and atolls 
discussed on former pages. The Pelew region is one of 
comparatively modern volcanic rocks and this renders local 
displacements a probability. The elevations are proof of 
local change of level; and likewise of a recent change in each 
case; for the top layer of the elevated reef-rock and all below 
it through the coral-made structure were completed previous 
to the uplift. They prove nothing as to the changes that 
were in progress when all this coral-rock was in process of 
formation. 

c. The occurrence of great numbers of large and small 
masses of coral rock, in some places crowded together, upon 
the western or leeward reef of the several Pelew islands, and 


1 First in 1868, Zeitschr. Wissensch. Zool. xiii. 558; additions in Die Philippi- 
nen und ihre Bewohner, Wiirzburg, 1869, and still later in his “ Animal Life ” pub- 
lished in Appleton’s International Scientific Series in 1881. 





OBJECTIONS TO THE THEORY OF SUBSIDENCE. 281 


of none on the eastern reef, is mentioned as evidence against 
subsidence and in favor of some elevation: because, Professor 
Semper says, the strongest wind-waves on the western side 
are too feeble to break off and lift on the reef so large masses, 
some of them (as his words imply rather than distinctly state) 
ten feet thick. 

But the difficulty does not exist in fact; for earthquakes 
may have made the waves. The region just west of the 
Pelews is one of the grandest areas of active volcanoes on 
the globe. It embraces the Philippine Islands, Krakatoa and 
other volcanic islands of the Sooloo Sea, Celebes, etc. The 
agents that could do the work were there in force. To the 
eastward, in contrast, lie the harmless islands of the Caroline 
Archipelago, mostly atolls, serving, perhaps, as a breakwater 
to the Pelews. 

Professor Semper has other steps in his theory which are 
considered beyond. 

The idea of elevation as a requisite is not suggested by 
the atolls of the ocean. In the Paumotu Archipelago, cover- 
ing four hundred and fifty thousand square miles, the nearly 
uniform height of the land, eight to ten feet, in the seventy 
to eighty atolls, with the shore platform a hundred yards or 
more wide near third tide-level, does not favor the assump- 
tion that elevation had put them into their present uniform 
positions. 

1. The talus-theory. — Mr. John Murray, one of the able 
naturalists of the Challenger Expedition, has proposed the 
following theory: that shore-reefs extend themselves out 
within coral-growing depths on the basement of debris de- 
rived from the growing margins. 

The fact that reefs widen by the process here mentioned 


when subsidence ceases is recognized in the author’s Expedi- 


282 CORALS AND CORAL ISLANDS. 


tion Report; but that this is the means of attaining great 
width and thickness with no aid from subsidence is the new 
view of Mr. Murray. The method has been described in the 
words: Reefs grow out on their own talus. The view is 
supported by Dr. Guppy, Mr. A. Agassiz, and others. 

a. Mr. Murray’s observations were made at Tahiti. He 
reports the following facts derived from soundings off north- 
ern Tahiti, made under his supervision and that of the sur- 
_veying officer. 

Along a line outward from the edge of the barrier reef 
there were found: (1) for about 250 yards, a shallow region 
covered partly with growing corals, which deepened seaward 
to 40 fathoms; (2) for 100 yards, between the depths of 40 
and 100 fathoms, a steeply but irregularly sloping surface 
which commenced with a precipice of 75° and had a mean 
angle exceeding 45°; then (3) for 150 yards a sloping bot- 
tom 50° in angle; (4) then a continuation of this sloping 
surface, diminishing in a mile to 6°, at which distance out 
the depth found was 590 fathoms (3,540 feet). Over the 
area (2), or the 100 yards between 40 and 100 fathoms, the 
bottom was proved to be made of large coral masses, some of 
them “20 to 30 feet in length,” along with finer debris; out- 
side of this, of sand to where the slope was reduced to 6°; 
and then of mud, composed ‘of volcanic and coral sand, 
pteropods, pelagic and other foraminifers, coccoliths, etc.” 

These observations have great significance. They show 
(1) that the feeble currents off this part of Tahiti carry little 
of the coral debris in that direction beyond a mile outside of 
the growing reef; (2) that a region of large masses of coral 


1 Dr. Archibald Geikie gives in his Presidential Address before the Royal 
Physical Society of Edinburgh in 1883 (Proc. viii. 1, 1883) a section of the soundings 
“on a true scale, vertical and horizontal,” and in it the upper steepest part of this 
100 yards has a slope of about 75°. 








OBJECTIONS TO THE THEORY OF SUBSIDENCE. 283 


rock and finer material occurs at depths between 240 and 
600 feet; (3) that, a mile out, the bottom has the slope 
nearly of the adjoining land, and in this part is covered with 
the remains of pelagic life. i 

- From the second of these facts, —the great accumulation 
of coral blocks below a level of 240 feet, — Mr. Murray draws 
the conclusion that, in the making of fringing, barrier, and 
atoll reefs, the widening goes forward (a) by making first 
upon the submarine slopes outside of the growing reef a 
pile of coral debris up to the lower limit of living reef-corals ; 
and then (6) by building outward upon this accumulation as 
a basement. 

b. But these observations fail to prove that an accumu- 
lation of coral blocks over the slopes below the reef-coral 
limit is a result of the process appealed to. In truth they 
set aside his conclusion by making the fact of a subsidence 
unquestionable. 

That belt of coarse debris — including “ masses 20 to 30 
feet” long — was found over the steeply sloping bottom at 


below the limit of forcible wave-action. They are depths 
where the waters, however disturbed above by storms, have 
no rending and lifting power, even when the bottom is grad- 
ually shelving; depths, in this special case, against a slope 
which for 100 yards is 75° in its upper part, and in no part 
under 45°, the vertical fall being 360 feet in the 100 yards. 
Strokes against the reef-rock thus submerged, and under such 
conditions, would be extremely feeble. Waves advancing up 
a coast, whether storm-driven waves or earthquake waves, 
do little rock-rending below the depth to which they can bare 
the bottom for a broadside plunge against the obstacle before 
them, although the velocity gives them transporting power 


/ . 


depths between 240 and 600 feet. These depths are far ~ 


284 CORALS AND CORAL ISLANDS. 


to a greater depth. It is the throw of an immense mass of 
water against the front, with the velocity increased by 
the tidal flow over a shelving bottom,—the rate some- 
times amounting, according to Stevenson, to 36 miles an 
hour or 52°8 feet a second, — together with the buoyant 
action of the water, that produces the great effects. 

A vertical surface below the sea-level of 20 feet made 
bare for the broadside stroke is probably very rarely ex- 
ceeded even in the case of earthquake-waves; and with 
storm-waves, or recorded earthquake-waves, the displace- 
ment of the water at a depth of 240 feet would be at 
the most only a few inches. I saw on atoll reefs no up- 
thrown masses of coral rock over ten feet in thickness and 
twenty feet in length or breadth. It is therefore plainly 
impossible that such a belt of debris should have been made 
at its present level, or even at a depth of twenty feet; and 
hence the debris affords positive proof of a large subsidence 
during some part of the reef-making era. 

ce. But if such accumulations of great blocks cannot result 
from the dropping of masses from the edge of the reef, the 
facts show that fine coral debris may be spread over the 
bottom for a mile outside of the surface reef, and contrib- 
ute thereby toward a rising basement. But the expression 
that they grow out on their own talus is wholly fallacious ; 
for there is nothing visible that is of this nature. Outside of 
the emerged reef there is a region of growing corals, as has 
been explained, and spots and areas of sand exist among the 
luxuriant groups and plantations. Outside of this there is 
more or less of deeper pelagic life and thinner deposits of 
sand from the reef; but nowhere a talus of debris. The sea 
makes debris as the waves and inflowing tides move toward 
the shores; but the action is almost. wholly landward, trans- 





OBJECTIONS TO THE THEORY OF SUBSIDENCE. 285 


porting nearly all the debris over the emerging reef and into 
the inner channels or lagoons; little is carried off and dropped 
in the deeper waters outside. The condition is like that on 
other coasts which the sea is extending. But there is this 
difference that the corals, the chief source of the debris, have 
a hold on the bottom because attached species, and the cal- 
careous sands are easily cemented; so that the reef they 
make becomes a solid bank. And further, the waves and 
tidal waters have this great area of reef and inner channels 
to receive the sands they distribute. Little debris reaches 
the outer slopes; and the most of this little finds lodgment 
among the corals within coral-growing depths. 

The Florida reefs well illustrate the facts. Off the great 
emerged banks there is, first, the region of growing corals, 
called the “ Florida reefs,” half a mile to a mile wide, and 
one to nine fathoms deep, where some spots of reef reach the 
surface ; and outside of this, on the south, there is the Pour- 
talés plateau at depths down to two hundred and fifty fath- 
oms, growing pelagic species. There is no talus. Some 
debris drops over the plateau; yet, as Mr. Agassiz’s figure 
shows, the amount is very small. The Florida Bank gets 
nearly all, and thus it has derived its height and extent. 

d. The view which Mr. Agassiz sustains that the Florida 
corals grew out on a basement made by pelagic growth or on 
the inner part of the Pourtalés plateau, and thus extended 
the great bank, has more to sustain it than the talus-theory. 
With only the facts that are open to view in the Florida seas 
no other explanation would be thought of. But there is one 
difference between banks over such a bottom and those in- 
creasing through a subsidence. With the former, the slope 
of the bottom would usually be gentle. While the explana- 
tion may answer as well as the subsidence-theory for the 


286 CORALS AND CORAL ISLANDS. 


Florida Bank, it does not appear to suffice for the Eastern 
Bahamas, with their steep submarine slopes toward the ocean, 
or for the Grand Cayman and other reefs southwest of Cuba, 
or for the great majority of the atoll and barrier reefs of 
the Pacific. It is, however, not yet proved to be true for 
Florida. 

e. By Darwin’s theory, the growing reef increases its 


thickness as the slow subsidence progresses; and the inside ~ 


channel, so common a feature, is a consequence in the way 
that has been explained. But by the Murray method, after 
the outer edge of the reef has a thickness equalling the 
range in depth of reef-corals for the region, the reef, as it 
further enlarges, keeps extending out into deeper and deeper 
water. But the width cannot be doubled without using three 
times as much material as for the preceding part in case the 
slope of the bottom is unchanged and the coast is a straight 
line, and much more than three times if the coast line is con- 
vex like that of Tahiti, and still more if the slope of the 
bottom increases. With the feeble amount of debris to be 
had, the rate of growth would therefore be extremely slow, — 
not over one hundredth of that for the first one thousand feet. 
During all the long era of this extension the waves and tidal 
movements and winds would be at their usual work, throw- 
ing the chief part of the debris on the reef and beyond into 
the channel; and thus the channel, the process being aided 
by its own growing corals, would be sure to become filled 
and nearly obliterated; for it is ever receiving, and has no 
chance to increase its capacity or relieve itself of the debris 
received. 

Among examples of a reef undergoing such an extension 
where there was little or no subsidence, is that of the north 
side of Upolu, of the Samoa Group, which the author’s Expe- 





OBJECTIONS TO THE THEORY OF SUBSIDENCE. 287 


dition Report describes as nearly a mile wide and yet having 
no channel deep enough for a canoe. The whole amount of 
subsidence estimated in the Report is one to two hundred feet 
— and whatever the amount it may have long since ceased. 
The same conclusion comes from the Florida Bank. The reef 
of Tahiti, on the contrary, is far as possible from such a con- 
dition. Its channel is partly a ship channel, as already de- 
scribed; and such deep waters are common within Pacific 
barrier reefs. 

The thick coral-made beds reached by artesian borings on 
Oahu (see Appendix), regarded as evidence of the subsidence 
of the island attending their formation, are explained by Mr. 
A. Agassiz on the Murray hypothesis: “the extension sea- 
ward of a growing reef, active only within narrow limits 
near the surface, which was constantly pushing its way sea- 
ward upon the talus formed below the living edge.” ? 

The above considerations may be deemed sufficient to set 
aside the suggestion; for it is only a suggestion, since no 
facts are mentioned in its support. The borings give it none. 
Some of them commence in the elevated reef which is the 
inner part of the fringing reef of the island, within three or 
four hundred yards of the mountain slopes, and go down 
through coral rock interruptedly for two hundred feet or more. 
There is no reason for regarding any part of it talus-made. 

jf. Although no sufficient reason is found for believing in 
talus-made basements, a characteristic by which they might 
be known if lifted into view may be mentioned. The debris 
is laid down on a sloping surface, whence the beds would 
have a corresponding pitch. The deposit would thus differ 
from ordinary coral formations and, too, from all great ex- 


amples of limestone strata of which we have knowledge. In 


1 Bulletin of the Museum of Comparative Zodlogy, 1889, xvii. No. 3. 


988 avs |, CORALS AND CORAL ISLANDS. 


the elevated coral island, Metia, two hundred and fifty feet 
high, the stratification is horizontal; the terraces of Christ- 
mas Island are horizontal; and the same is true of the ele- 
vated reefs of Cuba. 

If a satistactory decision in any case is not arrived at by 
the methods or criteria which have been described, there is 
the best of evidence to be had by artesian borings that shall 
go to the bottom and bring up a core six inches in diameter. 
It may thus be decided whether the limestone passed through 
is of reef-coral, or talus-debris, or partly or wholly of pelagic 
constitution." 

2. Courses of reefs and channels determined by marine 
currents.— The view has been urged by Lieut. E. B. Hunt, 
U.S. N. and Messrs. A. Agassiz, Murray, Semper, and Guppy 
that the positions of reefs off coasts are sometimes and gener- 
ally located by drift currents at considerable distances from 
coasts, and hence that the wide intervals of water inside of 
barrier reefs may result without aid from subsidence; and 
Dr. Guppy has urged, as remarked on another page, that 
atolls may be so shaped. 

a. The facts, presented by Lieutenant Hunt, and more 
fully by Mr. Agassiz with regard to the effects of the eddy 
current of the Gulf Stream have been mentioned on page 207. 
They show that coral reefs may be elongated, and also that 


1 The kind of submarine slopes to be ordinarily looked for off coral reefs is 
ilustrated by the Challenger soundings. And it is interesting to note that the facts 
sustain instead of correcting those announced by earlier observers. Beechey and 
Darwin make the mean slope about 45°, and my Report says 40° to 50°. I have 
assumed for the slope of the bottom outside of the reef-limit the same angle as for 
the surface-slope of the island just above the water-level ; 5° to 8° off Tahiti, of 
which 5° is accepted as most correct, and 3° to 5° off Upolu; and the assump- 
tion as regards Tahiti is sustained by the Challenger soundings. My Report 
states (from the Expedition surveys) that off Upolu, the bottom “loses more and 
more in the proportion of coral sand till we finally reach a bottom of earth,” and 
introduces this as an argument against the indefinite drifting of coral sands into the 
deep ocean; and this argument the Tahiti soundings sustain. 





OBJECTIONS TO THE THEORY OF SUBSIDENCE. 289 


inner channels may be made, by the drifting of coral sands. 
The action with coral sands is essentially the same as with 
other sands; and illustrations of this drifting process occur 
along the whole eastern coast of North America from Florida 
to Long Island. We there learn that drift-made beaches run 
in long lines between broad channels or sounds and the ocean ; 
that they have nearly the uniform direction of the drift of 
the waters, with some irregularities introduced by the forms 
of the coast and the outflow of the imner waters which are 
tidal and fluvial and have much strength during ebb tide. 
The easy consolidation of coral sands puts in a peculiar feature, 
but not one that affects the direction of drift accumulation. 

b. The great barrier reef off eastern Australia, a thou- 
sand miles long, has some correspondence in position to the 
sand-reets off eastern North America. But it is full of irreg- 
ularities of direction and of interruptions, and follows in no 
part an even line. In the southern half, it extends out one 
hundred and fifty miles from the coast and includes a large 
atoll-formed reef; in the northern half, the barrier, while 
varying much in course, is hardly over thirty miles from the 
land. There is very little in its form to suggest similarity 
of origin to the drift-made barriers of sand. 

c. In the Pacific Ocean, the trends of many of the coral 
island groups, and of the single islands, do not correspond 
with the direction of the oceanic currents, or with any eddy 
currents, except such as are local and are determined by 
themselves. 

Near longitude 180°, as the map of the Central Pacific 
(Plate IX.) illustrates, the equator is crossed by the long 
Gilbert (or Kingsmill) Group, at an angle with the meridian 
of 25° to 30°, and not in the direction of the Pacific current 
which is approximately equatorial. This obliquely crossing 

19 


290 CORALS AND CORAL ISLANDS. 


chain of atolls is continued northward in the Ratack and 
Ralick Groups (or the Marshall Islands), making in all a 
chain over 1,200 miles long; and, adding the concordant 
Kllice Islands on the south, and extending the Ratack line to 
Gaspar Rico, its northern outlier, the chain is nearly 2,000 
miles long. Nothing in the direction of the long range, ex- 
cepting local shapings of some of the points about the atolls, 

‘can be attributed to the Pacific currents. Moreover, none of 
, the diversified forms of atolls have any sufficient explanation 


~ in the drift process. 


Dr. Guppy urges the idea that banks of reef after reach- 
ing the surface are converted into atolls by means of the 
marine currents.’ In his last paper he speaks of the conver- 
sion after the island has been thrown up by the waves; in the 
earlier he appears to speak of elevation of some kind as 
necessary ; but the currents would act the same, whatever 
the means of reaching the surface. In the groups of atolls 
above referred to, neither the forms of the atolls nor the 
currents favor such an hypothesis. In the Paumotus, there 
are no currents of strength enough for work of this kind, 
as shown on page 296. He speaks of Keeling Atoll as a 
horse-shoe or crescentic island, and states that such forms are 
common among atolls. But the Keeling Atoll, according to 
the best maps, is an ordinary atoll, the lagoon nearly encircled 
with reef, and the application of such terms to it 1s wholly 
misleading. The shaping of a reef by the horse-shoe method 
of drifting and making an eddy to leeward which he describes, 
is wholly inapplicable to the ordinary atolls of the ocean; for 
there are no means for producing the result. 

d. Further, drifting by currents may make beaches and 


1 On the Solomon Islands, Proceedings of the Royal Society of Edinburgh, 
1885-1886, xiii. 857; On the Keeling Atoll, Nature, January, 1889, xxxix. 236. 





, 


OBJECTIONS TO THE THEORY OF SUBSIDENCE. 291 | 


inner channels whether subsidence is going on in the region 
or not, and are not evidence for or against either a move- 
ment downward or upward. Sandy Hook, the long, sandy 
point off the southern cape of New York harbor, has been 
undergoing (as the United States Coast Survey has shown) an 
increase in length, or rather variations in length, through the 
drifting of sands by an outside and an inside current; and 
this is no evidence that Prof. G. H. Cook is*wrong in his con- 


clusion that the New Jersey coast is slowly subsiding. 


3. Planting of corals on basements made and raised to the — 


right level by the growth of pelagic limestone or by other means. 
— The theory has been sustained by Mr. Semper, Dr. Rein, 
Mr. Agassiz, Mr. Murray, Dr. Guppy, and others, that since 
the growing calcareous deposits of the sea-bottom are slowly 
rising toward the surface by successive accumulations of the 
shells and other debris of pelagic species, they may have been 
built up locally in various regions of the deep seas (as they 
actually are now about some islands) until they were near 
enough to the surface to become next a plantation of corals; 
and that in this way, without any subsidence, atolls became 
common within the area of the tropical oceans. 

In support of this view, Dr. Guppy states that in elevated 
coral reefs of the Solomon Islands, one hundred to twelve 
hundred feet high, the coral reef rock forms a comparatively 
thin layer over impure earthy limestone abounding in fora- 
minifers, pteropods, and other pelagic organisms. Such ob- 
servations have great interest, but they only prove that in 
coral-reef seas corals will grow over any basis of rock that 
may offer where the water is right in depth and other cir- 
cumstances favor. They are not evidence against the subsi- 
dence theory,_but simply local examples under the general 
principle just stated. / Lr 


™. . : ” 


292 CORALS AND CORAL ISLANDS, 


The wide oceans are, however, wonderfully free from 
banks approaching the surface within one hundred fathoms. 
The Indian Ocean and the China seas afford exceptions. 
But the principal submerged banks of these waters have 
the lagoon-basins and raised borders of an atoll and thus 
bear, as has been explained (page 272), positive evidence of 
a former atoll existence and therefore of subsidence. They 
illustrate the fact ‘that the rate of subsidence has not always 
been within the narrow maximum limit that is favorable to 
‘the atoll, but sometimes has exceeded it to their destruction. 
The regions of such sunken atolls in the China seas and 
Indian Ocean have many spots of shallow soundings which 
were probably sunk at the same time. 

Mr. Murray observes that “the soundings of the ‘Tus- 
carora’ and ‘Challenger’ have made known numerous sub- 
marine elevations; mountains rising from the general level 
of the ocean’s bed at a depth of 2,500 or 5,000 fathoms up 
to within a few hundred fathoms of the surface.” But “a 
few hundred fathoms,” if we make “few” equal two means 
twelve hundred feet or more, which leaves a long interval 
unfilled.’ 

The atoll of the Tortugas, and others in the West Indies, 
are regarded by Mr. Agassiz as having a basement, up to the 
coral-growing limit, of pelagic limestone or of some other 
material. It may be so; but there is as yet no proof of it. 


1 The actual depths over the elevations in the “ Tuscarora” section between the 
Hawaiian Islands and Japan, numbering them from east to west are as follows: (1) 
11,500 feet; (2) 7,500 feet; (3) 8,400 feet; (4) 12,000; (5) 9,000 (this seven miles 
west of Marcus Island) ; (6) 9,600 feet. Whether ridges or peaks, the facts do not 
decide; probably the former. No. 1 has a base of 185 miles with the mean east- 
ward slope 40 feet per mile (= 1: 132) and the westward 128 feet per mile. No. 2 
has a breadth of 396 miles, with the mean eastern slope mostly 37 feet per mile, 
but 51 feet toward the top, and the westward 55 feet per mile (1:96). No.3 
was the narrowest and steepest, it being about 100 miles broad at base, and having 
the mean eastern slope 192 feet per mile and the mean western 200 feet. 


OBJECTIONS TO THE THEORY OF SUBSIDENCE. 293 


The basement, according to Mr. Murray, may be a vol- 
-eanic cone or a submerged mountain-peak at the proper level; 
and if too low, it may be raised up to it by pelagic life-relics ; 
if too high, like the “emerged volcanic mountains situated in 
the ocean basins,’ it may be razed by abrading agencies to 
the water-level and finally lowered by the waves and cur- 
rents to the needed depth. It is true that the basement for 
growing corals may be anything that has the proper depth. 
But this shaving off of a mountain and carrying the erosion 
to a few fathoms below the sea without a change of level is 
beyond physical possibility. Land-waters cut out valleys or 
gorges, and indefinite time would be required for the final lev- 
elling. Waves are too shallow in their action, the rising, tidal 
waters too protective, and the rocks under a cover of water too 
resisting, for the marine part of the degradation. 

But this making of atolls and barrier reefs on banks sub- 
merged at the right distance without submergence, encoun- 
ters a fatal difficulty wherever there are deep lagoons and 
deep channels, which difficulty has not been overcome though 
various methods have been suggested. 

4. Lagoon basins and channels made largely by abrading 
and solvent action. It is urged, in agreement with Darwin, 
that the outer portions of reefs increase faster than the inner, 
owing to the purer water about them and the more abundant 
life for food; that the inner parts are not only at a disadvan- 
tage in these respects, but suffer also from coral debris thrown 
over them. Some writers have considered this of itself suffi- 
cient, saying that the edges of the bank will thus reach the 
surface, while the interior makes little progress; and so 
comes the lagoon basin. But the above authors add to the 
causes of unequal growth mentioned by Darwin the solvent 
and abrading action of the waters. 


294 CORALS AND CORAL ISLANDS. 


It is, hence, concluded by them that, under these conditions, 
the simple bank of growing corals may have a depression 
made at centre, which, as the process continues, will become 
a lagoon basin, and the reef, thereby, an atoll with its lagoon; 
that the atoll, so begun, may continue to enlarge through the 
external widening of the reef and the further action of cur- 
rent abrasion and solution within; or, in the case of fringing 
reefs, that the change may go on until the reef has become a 
barrier-reef with an inner channel and inner reefs. It is ad- 
mitted that subsidence may possibly have helped in the case 
of the deepest lagoons. 

Dr. Geikie expresses his opinion on the subject thus :— 
“ As the atoll increases in size the lagoon becomes proportion- 
ally larger, partly from its waters being less supplied with 
pelagic food, and therefore less favorable to the growth of 
the more massive kinds of corals, partly from the imjurious 
effects of calcareous sediment upon coral growth there, and 
partly also from the solvent action of the carbonic acid of the 
sea-water upon the dead coral.” The argument has been en- 
forced by observations on the solvent power of carbonic acid. 
But this is a point about which there is no ground for dispute. 

a. Mr. Semper gives examples of the effects of currents 
at the Pelew Islands, stating that by striking against or flow- 
ing by the living corals they make the reef grow with steeper 
sides and determine its direction, and urging that abrasion 
and solution have made, not only the deep lagoon-like chan- 
nels, but the deeper channels between the islands. He holds 
that in Kriangle, which he describes as a true atoll with no 
channel leading into the lagoon from the sea, that the lagoon 
may have been “the result of the action of currents on the 
porous soil during a period of slow upheaval.’* He says, 


1 Animal Life, pp. 269, 270. 


OBJECTIONS TO THE THEORY OF SUBSIDENCE. 295 


further, that the large channel in the main island of the 
group, “forty fathoms deep and many miles wide. . . . finds 
an easy explanation on the assumption of an upheaval ;”’ it 
became “ wider in proportion as the enclosed island, consist- 
ing of soft stone [tufa], was gradually eaten away, and during 
slow upheaval it would continue to grow deeper in propor- 
tion as the old porous portions of the reef and the rock in 
which it was forming were more and more worn down by 
the combined action of boring animals and plants, and of the 
currents produced by the tides and by rain.” ; Mr. Semper 
refers to the dead depressed tops of some masses of Porites 
near tide-level as the effects of the deposit of sediment over 
the top of the living coral and of erosion by the waves and 
exposure to rains while the sides continued to grow; and the 
fact is made an example on a very small scale of atoll-making. 
In addition, experiments on the solvent power of sea-water 
are appealed to. Examples of the action of currents, sedi- 
ment, boring species, and the solvent action of carbonic acid 
in the waters, are mentioned by Mr. Agassiz, in his excellent 
account of the “Tortugas and Florida reefs,’ and in his 
“Three Cruises of the Blake ;” but in his paper on the Coral 
Reefs of the Hawaiian Islands he concludes that the fact 
that “the majority of reefs are of great width goes to show 
also that solution alone is not active enough to remove great 
masses and form lagoons.” ! —_ 

b. The theory, if satisfactory, accounts not only for the 
origin of an atoll, but for the origin of atolls of all sizes, 
shapes, and conditions, and for great numbers of them in 
archipelagoes and chains; not only for channels through 
fringing reefs, like those that abrasion in other cases makes, 
but for all the irregular outlines of barriers, for the great 


1 Bulletin of the Museum of Comparative Zoology, 1ssd, vol. xvii. No. 3. 


296 CORALS AND CORAL ISLANDS. 


barriers reaching far away from any land, and for the posi- 
tions and indented coasts of the small included lands. Is it 
a sufficient explanation of the facts? 

c. The currents that influence the structure of reefs are: 
(1) the general movement or drift of the ocean, in some parts 
varying with seasonal variations in the winds; (2) the cur- 
rents connected with wave action and the inflowing tide over 
a shelving bottom; and (3) the currents during the ebb, flow- 
ing out of channels; together with (4) counter-currents. 
Each region must have its special study in order to mark out 
all the local effects that currents occasion. Such effects are 
produced whether a secular subsidence 1s in progress or not, 
and hence a particular review of the subject in this place is 
unnecessary. 

The shaping of the outside of the reef and the determina- 
tion of the width and level surface of the shore-platform are 
due chiefly to the tidal flow and the accompanying action of 
wind-waves as explained on preceding page. 

The current that accompanies the ebb is locally the strong- 
est. Owing to the great width of many barrier reefs and of 
the channels and harbors within them, the tide flows in over 
a wide region. At the turn in the tide the waters escape at 
first freely over the same wide region; but with a tide of 
but two or three feet. there is little fall before the reef — 
which lies at low tide level and a little above it — retards it 
by friction; and thus escape by the open entrances is in- 
creased in-amount and in rate of flow. The facts are the 
same in atolls where the lagoons have entrances.) 


1 The currents of the tropical Pacific Ocean are of very unequal rate in its dif- 
ferent parts, and very feeble in the Paumotu Archipelago and the Tahitian and Sa- 
moan regions. Captain Wilkes reports that in the cruise of the Expedition through 
the Paumotu Archipelago to Tahiti, a distance of a thousand miles, during a month 
from August 13 to September 13, 1839, the drift of the vessels was only seventeen 
miles; and that during fourteen days in the first half of October, between Tahiti 


OBJECTIONS TO THE THEORY OF SUBSIDENCE. 297 


d. Examples of massive corals having the top flat, or de- 
pressed and lifeless, while the sides are living, are common 
in coral-reef regions, wherever such corals are exposed to the 
deposition of sediment, and where they have grown up to the 
surface so that the top is bare above low tide. A disk of 
Porites, having the top flat and the sides raised (owing to 
growth) so as to give it an elevated border, is figured on 
Plate LV. of the author’s Report on Zodphytes. Many such 
were found in the impure waters of a shore reef at the 
Feejees. At Tongatabu one flat-topped mass of Porites was 
twenty-five feet in diameter; and both there and in the 
Feejees, others of Astraeids and Mzandrinas measured twelve 
to fifteen feet in diameter. 

Over the dead surfaces, as Mr. Semper observes, the coral 
may be eroded by the solvent action of the waters and espe- 
cially where depressions occur to receive any deposits; and 


and Upolu of the Samoan group, nearly eighteen hundred miles, the drift was only 
forty-three miles. 

The “Challenger,” on her route from the Hawaiian Islands to Tahiti, found, be- 
tween the parallel of 10° S. and Tahiti, “ the general tendency of the current west- 
erly, but its velocity variable; ”” between the parallel of 10° S. and 6° N., the direction 
was westerly with “the average velocity thirty-five miles per day, the range seven- 
teen to seventy miles per day,” the maximum occurring along the parallel of 2° N. 
Farther west, about the Pheenix group, the equatorial current, as described by Mr. 
Hague (loc. cit. p. 237) has “a general direction of west-southwest and a velocity 
sometimes exceeding two miles per hour.” At times it changes suddenly and flows 
as rapidly to the eastward. The drifting of the sands about Baker’s Island (in lati- 
tude 0° 13’ N., longitude 176° 22’ E.) has much interest in connection with this sub- 
ject of current action, and the facts are here cited from Mr. Hague’s.paper. The 
west side of the little island (1 xX 2 m. in area) trends northeast, and the southern 
east by north, and at the junction a spit of sand extends out. During the summer 
the ocean swell, like the wind, comes from the southeast, and strikes the south side; 
and consequently the beach sands of that side are drifted around the point and 
heaped up on the western or leeward side, forming a plateau along the beach two 
or three hundred feet wide, and eight or ten feet deep over the shore platform. 
With October and November comes the winter swell from the northeast, which 
sweeps along the western shore ; and in two or three months the sands of the pla- 
teau are all drifted back to the south side, which is then the protected side, extend- 
ing the beach of that side two or three hundred feet. This lasts until February or 
March when the operation is repeated. 


298 CORALS AND CORAL ISLANDS. 


boring animals may riddle the coral with holes or tubes. 
But generally the erosion is superficial; the large masses 
referred to showed little of it. Such dead surfaces in corals 
are generally protected by a covering of nullipores and other 
incrusting forms of life, and the crusts usually spread over 
the surfaces pari passu with the dying of the polyps. 

e. Every stream, says Mr. Semper (when explaining, as 
cited on a preceding page, the origin of the deep channel of 
the large Pelew Island, whose depth is “ 35 to 45 fathoms”’), 
‘has a natural tendency to deepen its bed.” But there is a 
‘limit to this action. The eroding or deepening power of a 
stream through abrasion and transportation is null, or nearly 
so, below the level of its outlet. A basin or channel 45 fath- 
oms (270 feet) deep. with an outlet of much less depth could 
not be deepened by such means nor protect itself from shallow- 
ing. The depth of the outlets is not stated except that they 
are said to be ship-channels. Moreover, with a tufa bottom, 
solution could not contribute to the removal, since carbonated 
waters, although decomposing the tufa, dissolve very little of 
its ingredients. An elevation in progress would result in 
making the channel a closed lake and finally dry land. 

For the same reason, the small atoll, Kriangle, having, as 
described, a closed lagoon, could have no deepening of the 
lagoon from abrasion by tidal currents or wave-action during 
the progress of an elevation. And if a lagoon have an out- 
let, the rapid current of the ebb would be confined to the 
narrow passage-way and a portion of the bottom near it; 
through the larger part of the lagoon, as in any other lake, 
the waters would have scarcely perceptible motion, and there- 
fore slight energy for any kind .of work. Hence a lagoon 
would lose very little by this means, and shallowing would 
go on unless there were great loss through the solvent action 





OBJECTIONS TO THE THEORY OF SUBSIDENCE. 299 


of the waters. An elevation would only hurry the shallow- 
ing and end in emptying the lagoon. 

f. Erosion through solvent action is promoted by the 
presence in the waters both of carbonic acid and organic 
acids. The material within reach of the tides or waves ex- 
posed to this action is dead corals and shells, or their debris, 
and bare coral rocks, occurring over: (1) the outer region of 
living corals and for a mile or so outside ; (2) the shore plat- 
form and the reef, bare at low tide, on which there is com- 
paratively little living coral; and (3) the lagoon basin. 
There is nothing in the material within the lagoon to favor 
solution more than in either of the other two regions; in 
fact, the platform and bare reef are most exposed to the 
action because of the small amount of living corals over 
them. The outside waters take up what they can through 
the carbonic acid they contain, and supply thereby the wants 
of the lime-secreting polyps, shells, ete., and carry on the pro- 
cess of solidification in the debris; the same waters move on 
over the atoll reef and take up more lime as far as the acid 
ingredient is present ; and then they pass to the lagoon for 
work similar to that outside, with probably a diminished 
amount of free carbonic acid, on account of the loss over the. 
reef-ground previously traversed. 

The lagoon-basin is not, therefore, the part of the atoll 
that loses most by solution, any more than by abrasion and 
transportation. The outer reefs suffer the most; and yet, if 
the island is not subsiding at too rapid a rate, they keep ex- 
tending and encroaching on the ocean, instead of wasting 
through the drifting into the ocean at large of calcium car- 
bonate in grains and solution, and the shore-platform also 
preserves its unvaried level notwithstanding the daily sweep 
of the tidal floods, and the holes that riddle its outer portion. 


300 CORALS AND CORAL ISLANDS. 


The remark: “It is a common observation in atolls that 
the islets on the reefs are situated close to the lagoon shore;” 
such “ facts point out the removal of matter which is going 
on in the lagoons and lagoon channels,” I know nothing to 
sustain. The width of the shore-platform on the seaward 
side is always greater than that on the lagoon side; but the 
outside shore-platform has its width determined by tidal and 
wave action, and this action is powerful on the ocean side, 
and feeble on the lagoon side; it produces a high coarse 
beach on the outside as the inner limit of the platform, and 
a finer, lower and much more gently sloping beach on the 
inside. The amount of erosion is far greater, as it should be, 
on the side of the powerful agencies. 

g. The loss to the lagoon by abrasion and solution is re- 
duced toa minimum, nm the majority of atolls, by the absence 
of lagoon entrances, which leaves them with only concealed 
leakage passages for slow discharge. 

Nine tenths of atolls under six miles in length (or in 
longer diameter), half of those between six and twenty 
miles, and the majority of all atolls in the Pacific ocean, 
have no entrances to the lagoon a fathom deep; and the 
larger part of those included in each of these groups have 
no open entrances at all. 

For evidence on this subject, reference may be made to 
the Wilkes Expedition Hydrographic Atlas. This atlas con- 
tains maps of nearly sixty coral islands from the surveys of 
its officers, drawn on a large scale — one or two miles, rarely 
four, to the inch. 

Out of the number, nine, ranging from 13 to 3 English 
miles in the longer diameter of the reef, have no lagoon, but 
only a small depression in its place; two of these take in 
water at high tide, and the rest are dry. 


OBJECTIONS TO THE THEORY OF SUBSIDENCE. 301 


Of those under six miles im length having lagoons, seven- 
teen in number, sixteen are represented as having no en- 
trances to the lagoon at low tide; and the one having an 
entrance is 5X4 miles in size. The smallest is about a mile 
in diameter. 

Of those that are six miles or over in length, twenty-nine 
in number, seventeen have channels and twelve have none. 
Those having channels are mostly over ten miles in length. 
A list of them is here given with their sizes, and also the 
proportion of the reef around the lagoon which is under 
water above one third tide, and bare at low tide,—a feature 
of much interest in this connection : — | 

ELLIcE Group. — Depeyster’s, 6X6 m.; three fourths of 
the encircling reef bare. llice’s, 9X5 m.; three fourths bare. 

GILBERT Group. — Apia, 17X7 m.; half bare. Tarawa, 
io m.; hali bare. ‘Taritari, 18X11 m.; two thirds bare. 
Apamama, 12X5m.; half bare. Tapateuea, west side mostly 
submerged. 

MarsHaLut Istanps (northern). — Pescadores, 108 m. ; 
four fifths bare. Korsakoff, 26 m.; four fifths bare. 

Paumotus. — Peacock, 15X7 m.; nearly all wooded. 
Manhii, 13X5 m.; nearly all wooded. Raraka, 6X9 m.; 
three fourths wooded. Vincennes, 139 m.; mostly wooded. 
Aratica, 18X11 m.; three fifths bare. Tiokea, 184 m.; two 
thirds wooded. Kruesenstern’s, 16X10 m.; mostly wooded. 
Dean’s (or Nairsa), 55X18 m.; half or more bare. 

h. The absence of open channels in so large a proportion 
of lagoons, and especially in lagoons of the smaller atolls, is 
fatal to the abrasion-solution theory. The method of enlarg- 
ing atolls through currents and solution can act only feebly, 
if at all, where waters have no free outlet ; and this is emi- 
nently so with the smaller atolls which have been assumed 


4 , 


302 CORALS AND CORAL ISLANDS. 


by the theory to be most favorable in purity of water and in 
abundant life for progress; if the small cannot grow, the 
large lagoons cannot be made from them by the proposed 
method. 

Reverse the method, letting the large precede the small, 
and then, under the subsidence theory, we have a consistent 
order of events. We have large atoll reefs with several large 
entrances (like the great barrier reef about a high island in 


trances concurrently narrowing through the growing corals 
and the consolidating debris, in spite of the efforts of abra- 
sion and solution to keep them open and make them deeper ; 
and, afterwards, the atoll becoming still smaller until the 
entrances close up; and finally the lagoon-basin reduced to 
a dry depression with nothing of the old sea-water remaining 
except, perhaps, some of its gypsum. 

i. Instead of small lagoons having the purest waters, the 
reverse is most decidedly and manifestly the fact, and this 
accords with the reversal in the history just suggested. Since 
atolls of middle and larger size commonly have one third 
to two thirds of the encircling reef covered with the sea at 
one third tide, making the ocean and lagoon for more than 
half the time continuous, the large lagoon in such a case has 
as pure water as the ocean, and commonly as good a supply 
of food-life, and sometimes as brilliant a display of living 
corals. But in the smaller atolls, the area of the lagoon has 
little extent compared with the length and area of the encir- 
cling reef ; coral sands and other calcareous material conse- 
quently have possession of the larger part of the bottom, 
and the waters, since they are less pure than those outside, 
contain fewer and hardier kinds of corals and less life of 
other kinds. They are exposed, also, to wider variations of 


49 
this and other respects) gradually contracting, and the ont 


Aw 
- 


} 


4 


OBJECTIONS TO THE THEORY OF SUBSIDENCE. 303 


temperature than the outer, with injury to many species, and, 
at lowest tides, may become destructively overheated by the 
midday sun, as many a plantation of corals with dead tops 
for a foot or more bears evidence. In the smallest atolls, 
the lagoons are liable also to alternations of excessive saltness 
from evaporation and excessive freshness from rains, and con- 
sequently no corals can grow inside, though still flourishing 
well in the shallow sea about the outer reef. The above are 
the facts, not the suggestions of theory. __ 

j. We read: “So great is the destructive and transport- 
ing influence of the sea under the combined or antagonistic 
working of tides, currents, and wind-waves that the whole ! 
mass of the reef, as well as the flats and shoals inside, may 
be said to be in more or less active movement.’* ‘This 
description of the Tortugas reefs is not applicable to the 
atolls of the Pacific nor to the Tortugas. Notwithstanding 
the testimony of Captain Beechey and others about occa- 
sional catastrophes — which are mostly catastrophes to the ~ 
islets and banks within the lagoons—I was led to look 
upon a coral island as one of the most stable of structures. 
Through the wind-made and tidal movements, the loose sands 
are drifted along the shores and over the reef; edges of the 
reef are broken off in gales or by earthquake waves; and 
occasionally a mushroom islet in the lagoon, where growing 
corals are not compacted by wave-action, is overthrown by 
the same means; but beyond this. the structure is singularly 
defiant of the encroaching waters. Earthquakes may bring 
devastation ; and so they may to other lands. 

5. Lagoon-basins made by caving in. — Mr. Fewkes has 
suggested” that the lagoon-basin of the Bermudas may have 


1 Dr. Geikie’s Address, p. 23. 
2 Fewkes, Proceedings of the Boston Society of Natural History, 1888, p. 518. 


304 CORALS AND CORAL ISLANDS. 


been made by the caving in of caverns with which the reef- 
rock is supposed to have been honey-combed, but does not 
apply the theory to all other atolls. The view is opposed 
by Mr. Heilprin, after a more recent study of the islands. 
There is nothing to sustain it among the Pacific atolls 
that were visited by the author. Ordinary atolls have no 
caves ; for caves in reef-limestones are made only after their 
elevation. The constant effort and action of the sea upon 
submerged reefs is to fill up all cavities and consolidate all 
sands. In the drift-sand ridges or hills, however, they may 
be formed without an elevation of the island; but these do 


é 


not exist in the region of the lagoon-basin. « ., //..: 


; 


sie , % t t 


As a further reply to the arguments adverse to Darwin’s 
theory from the West Indian seas, the following facts are 
here added. 

The arguments from the Florida and West Bahama reefs 
in favor of no subsidence have more weight than for most 
coral-reef regions. They appear to indicate that any subsi- 
dence once in progress long since ceased; but they are far 
from proving that the reefs of the seas have been formed 
without help from subsidence. There is evidence that a 
great subsidence occurred during the coral-reef era, affording 
all that the Darwinian theory demands. 

In a valuable paper by Mr. Alexander Agassiz, published 
in 1879 in the Bulletin of the Museum of Comparative Zodl- 
ogy,” the author points out that the South American conti- 
nent, in comparatively recent geological times, had connection 
with the West India islands through two lines: (1) one along 
a belt from the Mosquito Coast to Jamaica, Porto Rico, and 


1 Heilprin, Bermuda Islands, p. 44. 1889. 
2 An abstract of the paper is contained in the American Journal of Science, 
1880, xviii. 230. 











OBJECTIONS TO THE THEORY OF SUBSIDENCE. 305 


Cuba; and (2) the other through Trinidad to Anguilla, of 
the Windward Islands. He sustains the conclusion by a 
review of the soundings made by the steamer “ Blake” under 
the command of J. R. Bartlett, U. S. N., and a consideration 
of the facts connected with the distribution of marine and ter- 
restrial species. As the soundings show, the former of the 
two connections requires for completeness an elevation of the 
region amounting to 4,060 feet over the part south of Jamaica, 
4,830 feet between Jamaica and Hayti, and 5,240 feet between 
Hayti and Cuba. The other line of connection requires an 
elevation of 3,450 feet. An open channel, as he observes, 
would thus be left between Anguilla and the Virgin Islands, 
where there is now a depth of 6,400 feet. The close relations 
in the existing fauna of the Gulf to that of the Pacific waters 
prove that it continued to be a salt-water gulf through the 
era of elevation. 

Mr. Agassiz infers that the connection of the West India 
islands with South America existed before the Quaternary era. 
But there are other facts which seem to prove that it was con- 
tinued into, or at least was a fact, in the Quaternary. 

The opinion as to a connection of the Windward Islands 
with South America in the Quaternary was presented by 
Prof. E. D. Cope in 1868 (and earlier, as he states, by Pomel), 
on the ground of the discovery in the caves of Anguilla of a 
species of gigantic rodent related to the chinchilla, as large 
as the Virginia deer, and nearly equalling the Quaternary 
Castoroides of Ohio. Further, De Castro, as cited by Dr. J. 


1 Proceedings of the Academy of Natural Sciences of Philadelphia, 1868, 313, 
and of the American Philosophical Society, Philadelphia. 1869, 183; also Smithsonian 
Contributions to Knowledge, 30 pp. 4to with 5 plates, Washington, 1883. The last 
paper (prepared in 1878) contains descriptions of the following species from the 
Anguilla bone-cave: Amblyrhiza inundata Cope (the large rodent announced in 
1869), A. quadrans Cope, A. latidens Cope, an Artiodactyl, apparently of the Bovi- 
de and a little smaller than Ovis aries. With them was obtained an implement 
(“a spoon-shaped scraper or chisel ”?) made of the lip of the large Strombus gigas. 

20 


306 CORALS AND CORAL ISLANDS. 


Leidy in his “ Mammalian Fauna of Dakota and Nebraska,” 
1869, announced, in 1865, a gigantic sloth of the Quater- 
nary, from Cuba, which he referred to the genus MMegalonyz, 
and Dr. Leidy named Megalocnus rodens, proving a Quater- 
nary connection between the continent and Cuba. 

The fact of an elevated condition of the region sufficient 
to make Cuba and Anguilla part of the continent during the 
earlier Quaternary, 1f not in the Pliocene also, is thus made 
quite certain. This is fully recognized by Wallace. Such a 
condition could hardly have existed without a large elevation 
also of Florida, though probably not, as Mr. Agassiz holds, 
to the full amount of the depression between it and Cuba — 
nearly 5,000 feet — because Cuba is most closely related in 
fauna to South America. The subsidence which brought the 
region to the present level was consequently within the coral- 
reef period. It is hence hardly to be doubted that the mak- 
ing of the Florida, Bahama, and other West India coral reefs 
was going on during the progress of a great subsidence. 
None of the facts mentioned by observers are opposed to 
this view. 

On this point Mr. Heilprin says :? — 

“Dr. Supan, in reviewing Professor Dana’s paper in the 
American Journal of Science for 1885,’ criticises the views 
relative to subsidence in the Floridian region, since it is 
claimed that even if direct connection did exist between the 
West Indian Islands and the southern continent, there is no 
proof that this connection extended northward to the North 


American continent ; and he further denies — without, how- 





ever, giving any reason for his denial —that there ever was 
any (Quaternary ?) connection between the West Indies and 
1 Geographical Distribution of Animals, ii. 60, 78. 


2 «The Bermudas,” 227. 
$ Petermann’s Mittheilungen, vol. xxxii. pl. 1; Litteraturbericht, p. 5. 1886. 


OBJECTIONS TO THE THEORY OF SUBSIDENCE. 307 


North America. This notion is probably based upon the old 
idea (advanced by L. Agassiz and Le Conte) of the making 
of the Floridian peninsula, in which no movements of either 
elevation or subsidence were supposed to have been involved. 
Since, however, this conception has proved to be a myth, 
there is no further reason, except so far as the case may be 
supported by fact, to adhere to the old views of continental 


(or oceanic) stability in this region. My own observations ‘ 


have conclusively proved a peninsula uplift as late as the 
Post-Pliocene period, and extending as far south as Lake 
Okeechobee. But I am by no means convinced, as I have 
elsewhere stated (in chapter on the Coral-reef Problem) that 
a nearly simultaneous subsidence did not take place in (and 
from) what are now known as the straits of Florida. The 
existence of such a subsidence (Lruch) is considered likely by 
Suess* who has paralleled it with (a supposed) similar occur- 
rence in the eastern basin of the Mediterranean. This view 
of the formation of the deep Gulf-channel, I must confess, 
appears to me far more captivating than that which ascribes 
it to the wash of the Gulf-current.” 

Mr. Heilprin brings forward, also, additional evidence of 
great weight. 

“But I believe direct evidence pointing to (although by 
no means proving) a former connection between the Floridian 
peninsula and the mainland is not wanting. In a paper on 
‘The Value of the “ Nearctic,” as one of the Primary Zodlogi- 
eal regions,’ published in the Proceedings of the Academy of 
Natural Sciences of Philadelphia for 1882, I pointed out cer- 
tam facts in favor of considering the lower portion of the 
peninsula as part of the Neotropical rather than the Nearctic 
realm; more recent zodlogical researches have still further 


1 Antlitz der Erde, vol. i. 


308 CORALS AND CORAL ISLANDS. 


demonstrated the correspondence existing between this south- 
ern fauna and that of the tract lying to the south. But more 
significant is the finding of the large assemblage of mamma- 
lian remains which have been lately brought to light from 
various parts of the peninsula. These have been determined 
by Dr. Leidy* to be the skeletal parts of the elephant, masto- 
don, llama, rhinoceros, tapir, Hippotherium, the sabre-toothed 
tiger (Machzerodus), Glyptodon, etc. Neither the Sabre-tooth 
nor the Glyptodon, both of which are so closely related to the 
commoner South American forms as to be barely distinguish- 
able from them, have heretofore been found in the south- 
ern United States. Of course they may yet be found, and 
indicate a passage over from South America by way of Mex- 
ico and the southern United States. But the great abund- 
ance of these remains on the Floridian peninsula, and their 
absence, either in whole or part, from the Gulf States, are 
facts which, so far as they go, point to a former direct land- 
connection across what is now an arm of the Gulf.” 

Concrusion. — With the theory of abrasion and solution 
to make lagoon-basins and the deep channels inside of barriers 
incompetent, and also that of current-drift, all the hypotheses 
of objectors to Darwin’s theory are alike weak ; for they have 
made these processes their reliance, whether appealing to a 
calcareous, or volcanic, or pelagic-growth, or mountain-peak 
basement for the structure. The subsidence which the Dar- 
winian theory requires has not been opposed by the mention 
of any fact at variance with it. nor by setting aside the argu- 
ments in its favor; and it has found new support in the facts 
from the “Challenger’s”’ soundings off Tahiti that had been 
put in array against it, and strong corroboration in the evi- 
dences of subsidence from the West Indies. 


1 Leidy, Proceedings of the Academy of Natural Sciences of Philadelphia, 
1884-1889. 





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’ wh bP’ 4 THE COMPLETED ATOLL. 309 


~ Darwin’s theory, therefore, still remains as the theory 

af that accounts for the origin of atolls and barrier islands, 

which is not true of any other that has been proposed. 
Fringing reefs and isolated coral-reef banks may form in 
shallow water within the growing depths of reef-making 
corals, and on any kind of bottom. But atolls, barrier-reefs, 
and coral formations of great thickness require, as a rule, the 
aid of slow subsidence, —as has been true for nearly all the 
thick rock formations over the continents. 


V.—THE COMPLETED ATOLL. 


The atoll, a quiet scene of grove and lake, is admirably 
set off by the contrasting ocean. Its placid beauty rises to 
grandeur when the storm rages, and the waves foam and 
roar about the outer reefs; for the child of the sea still rests 
quietly, in unheeding and dreamy content. This coral-made 
land is firm, because as has been already explained, it is liter- 
ally sea-born, it having been built out of sea-products, by the 
aid of the working ocean. And so with the groves: they 
were planted by the waves; and hence the species are those 
that can defy the encroaching waters, and meet the various 
conditions in which they are placed. The plants therefore 
take firm hold of the soil, and grow in all their natural 
strength and beauty. 

Only an occasional coral island has a completely encir- 
cling grove, and is hence a model atoll. But the many in 
which a series of green islets surround the lagoon are often 
but little less attractive, especially when the several islets pre- 
sent varied groupings of palms and other foliage. To give per- 
fection to the coral island landscape there ought to be, here 
and there, beneath the trees, a pretty cottage or villa, and 
other marks of taste and intelligence; and now and then a 


310 CORALS AND CORAL ISLANDS. 


barge should be seen gliding over the waters. As it is, the 
inhabitants are swarthy and nearly naked savages, having 
little about them that is pleasant to contemplate ; and their 
canoes with a clumsy outrigger to keep them right side up, 
as well as their thatched huts, are as little in harmony as 
themselves with Nature’s grace and loveliness. 

Where the islets of a coral reef are heaped up blocks of cor- 
al rock, blackened with lichens, and covered with barely 
enough of trailing plants and shrubs to make the surface green 
in the distant view, the traveller, on landing, would be greatly 
disappointed. But still there is enough that is strange and 
beautiful, both in the life of the land and sea, and in the his- 
tory and features of the island, to give enjoyment for many a 
day. 

The great obstacle to communication with a majority of 
atolls, especially the smaller, is the absence of an entrance to 
the lagoon, and hence of a good landing-place. In that case 
landing can be effected only on the leeward side, and in good 
weather; and best, when the tide is low. ven then, the sea 
often rolls in, so heavily, over the jagged margin of the reef, that 
it is necessary for the boat to take a chance to mount an in-go- 
ing wave and ride upon it over the line of breakers, to a stop- 
ping-place somewhere on the reef’ or shore-platform. 

Less easy is the return through the breakers, especially if 
the sea has risen during the ramble ashore. The boat, in or- 
der to get off again, would naturally take one of the narrow 
channels or inlets indenting the margin of the reef. But, 
with the waves tumbling in one after another, roughly lifting 
and dropping it, as they pass, and with barely room between 
the rocks for the oars to be used, there is a fair chance of its be- 
ing dashed against the reefs to its destruction, or thrown 
broadside to the sea and swamped under a cataract of waters. 


















































THE COMPLETED ATOLL. Lt 


If another boat with its crew were lying at the time off the 
reef, a line, carried to it through the surf by an expert swim- 
mer, might prove a means of rescue:—and so, in 1840, we 
safely reached our ship. To those approaching such a shore in 
a boat, prudence would give the advice—first, drop, some dis- 
tance outside of the breakers, a kedge or anchor, for aid both 
in landing on, and leaving, the reef. But the bottom off a cor- 
al island is often bad anchoring ground. And then, if the 
kedge thus planted holds firm, in spite of the jerking waves, 
well and good. If not 

The accompanying plate represents a scene on Bowditch or 
Fakaafo Island, sketched by Mr. A. T. Agate, one of the artists 
of the Wilkes Exploring Expedition, and copied from Volume 
V. of Wilkes’s Narrative of the Expedition. This island is 








FAKAAFO, OR BOWDITCH ISLAND. 


the easternmost of three small atolls, situated to the north of 
the Samoan or Navigator Group, near the parallels of 84°, 9°, 
and 93° S., and between the meridians of 171° and 1723° 


12, CORALS AND CORAL ISLANDS. 


(Su) 


and has already been described (p. 168). The grove of cocoa- 
nut trees contains the sacred or public house of the island— 
a well-made structure measuring fifty feet by thirty-five in 
length and breadth, and twenty feet in height. In front of 
the building stands the deity of the place, consisting of a 
block of stone fourteen feet high, enveloped in mats; and also 
near by, a smaller idol, partially covered with matting. In 
the left corner there is a young cocoanut palm—usually a more 
beautiful object than the full-grown tree. 

This island and the two others near it were among the 
few, perhaps the last, examples that remained until 1840, 
of Pacific lands never before visited by the white man. 
The people therefore were in that purely savage state which 
Captain Cook found almost universal through the ocean in 
the latter part of last century. A few words respecting our 
reception at this coral island, may not, therefore, be an im- 
proper digression. 

The islanders knew nothing of any other land or people: 
—an ignorance not surprising, since the lagoons of the group 
have no good entrances, and a nation cannot be great in nav- 
igation or discovery without harbors. As a consequence, our 
presence was to them like an apparition. The simple in- 
habitants took us for gods from the sun, and, as we landed, 
came with abundant gifts of such things as they had, to pro- 
pitiate their celestial visitors. They, no doubt, imagined that 
our strange ship had sailed off from the sun when it touched 
the water at sunrise, or sunset, and any child among them 
could see that this was a reasonable supposition. The king, 
after embracing Captain Hudson, as the latter states in his 
Journal (Wilkes’s Narrative), rubbed noses, pointed to the 
sun, howled, moaned, hugged him again and again, put a mat 
around his waist, securing it with a cord of human hair, and 















































































































































































































































































































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THE COMPLETED ATOLL. sail iiss 


repeated the rubbing of noses and the howling; and the mo- 
ment the captain attempted to leave his side, he set up again 
a most piteous howl, and repeated in a tremulous tone, ‘‘ Nofo 
ki lalo, mataku au,” ‘Sit down, I am afraid.” While thus 
in fear of us, they showed a great desire that their dreaded 
visitors should depart; some pointed to the sun, and asked 
by their gestures about our coming thence, or hinted to us to 
be off again. 

But with all their reverence toward their mysterious guests, 
they became after awhile quite familiar, and took advantage 
of every opportunity to steal from us. Our botanist gave his 
collecting-box to one of them to hold, and, the moment his 
back was turned, off the native ran, and a hard chase was re- 
quired to recover it—a most undignified run on the part of 
the celestial. 

While the men wore the maro, the equivalent of tight-fit- 
ting breeches six inches or less in length, the women were at 
tired in a simple bloomer costume, consisting solely of a petti- 
coat or apron, twelve to eighteen inches long, made of a large 
number of slit cocoanut leaves, and kept well oiled. Besides this 
they had on, as ornaments, necklaces of shell or bone. The girls 
and boys were dressed au naturel, after the style in the garden 
of Eden. These primitive fashions, however, were not peculiar 
to the group, being in vogue also in other parts of the Pacific. 

As a set-off against the geographical ignorance of these 
islanders, we may state that Captain Hudson and the best 
map-makers of the age knew nothing of the existence of Bow- 
ditch Island until he discovered it; and from him comes the 
name it bears, given in honor of the celebrated author of 
“ Bowditch’s Navigator” as well as of the translation of [a- 
place’s Mécanique Céleste. 

The annexed plate—also from Wilkes’s Narrative, Vol. V. 


314 CORALS AND CORAL ISLANDS. 


—represents a scene on Duke of York’s Island, another of the 
same solitary group of atolls. The view was taken on the 
lagoon side, and exhibits the placid lake, the border of verdure 
far away in the distance, and, near by, the margin of a native 
village beneath its cocoa-nut grove. A few young plants of 
the Pandanus stand along the point. The houses are like 
those of other islands to the west and northwest. The point 
in front of the village is one of three small quays, two feet out 
of water. The house, resting partly upon it and partly on 
poles in the water, and thatched with leaves of the Pan- 
danus, was apparently a shelter for canoes and fishing-tackle. 
The Gilbert Group affords an example of a less isolated 
coral-island people. A. beautiful view representing a part of 
the village of Utiroa, on Drummond’s Island, is contained in 
the same volume of Wilkes’s Narrative with the preceding. 
The public-house of the island is even larger than that on 
Bowditch’s Island, measuring one hundred and twenty feet in 
length, forty-five feet in width, and forty in height to the 
ridge-pole. This island, unlike the Duke of York’s, was 
densely peopled, and, owing apparently to the scant supply of 
fish and vegetables thus occasioned, many of the natives were 
afflicted with leprosy, and also had bad teeth, both circum- 
stances unusual for the Pacific. Lean in body and savage in 
look and gesture, they strangely contrasted with their fat, jolly 
kinsmen on some of the more northern islands of the same 
group. An old, fat chief who came from one of these islands 
to the ship’s side in his canoe was actually too large to have 
reached the deck except by the use of a tackle. It was evi- 
dent that infanticide—a necessity according to their system 
of political economy—was more thoroughly practised than on 
Drummond’s Island, and that the population was thus kept 
from becoming uncomfortably numerous. The obesity was 


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THE COMPLETED ATOLL. 1) 


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probably owing to their having nothing to do, and plenty, 
in the vegetable way, to eat; for these equatorial islands, 
somewhat elevated, as elsewhere observed, are unusually pro- 
ductive for atolls,—just the place for a voluptuous barbarian. 
The people on Drummond’s Island were great thieves, and 
knew the pleasures of a cannibal feast. Without metals, or 
any kind of hard stone, they make, out of the teeth of the 
sharks caught about the reefs, a sharp, jagged edging for long 
knives, swords and spears; and the women, jealous of one an- 
other, sometimes, as Mr. Hale says, carry about with them for 
months a small weapon of shark’s teeth concealed under their 
dress, watching for an opportunity to use it; and desperate 
fights sometimes take place. The same author mentions, also, 
some good points in them: observing that the women are, for 
the most part, better treated than is common among uncivil- 
ized people; that the men do the hard out-door work, while 
the women clear and weed the ground, and attend to the do- 
mestic duties that naturally fall to them. ‘Custom also re- 
quires that when a man meets a female he shall pay her the 
same mark of respect that is rendered to a chief, by turning 
aside to let her pass,’—a rule that probably does not al- 
ways hold in practice. He adds: “The word manda sig- 
nifies among the Gilbert Islanders a man thoroughly accom- 
plished in all their knowledge and arts, and versed in every 
noble exercise; a good dancer, an able warrior, one who has 
seen life at home and abroad, and enjoyed its highest excite- 
ments and delights—in short, a complete man of the world. 
In their estimation this is the proudest character to which any 
person can attain; and such a one is fully prepared to enter, 
at his death, on the highest enjoyments of their elysium.” 
Thus much for the human productions of coral islands. 


Coral islands are exposed to earthquakes and storms like 


316 CORALS AND CORAL ISLANDS. 


the continents, and occasionally a devastating wave sweeps 
across the land. During the heavier gales, the natives some- 
times secure their houses by tying them to the cocoanut trees, 
or toa stake planted for the purpose. A height of ten or 
twelve feet, the elevation of their land, is easily overtopped 
by the more violent seas; and great damage is sometimes 
experienced. The still more extensive earthquake-waves, 
such as those which have swept up the coast of Spain, Peru, 
and the Sandwich Islands, would produce a complete deluge 
over these islands. We were informed by both Grey and 
Kirby that effects of this kind had been experienced at the 
Gilbert Islands; but the statements were too indefinite to 
determine whether the results should be attributed to storms, 
or to this more violent cause. 

But while coral islands have their storms, the region in 
their vicinity is generally one of light winds and calms, even 
when the trades are blowing strongly all around them. The 
heated air which rises from the islands lifts the currents to a 
considerable height above the island. J.D. Hague mentions 
that on Jarvis’ and the two neighboring islands, under the 
equator, near 180° in longitude from Greenwich, he “ often 
observed the remarkable phenomenon of a rain squall ap- 
proaching the island, and, just before reaching it, separat- 
ing into two parts, one of which passed by on the north, the 
other on the south side, the cloud having been cleft by the 
column of heated air rising from the white coral sands.” 

An occasional log drifts to the shores, and at some of the 
more isolated atolls, where the natives are ignorant of any 
land but the spot they inhabit, they are deemed direct gifts 
from a propitiated deity. These drift-logs were noticed by 
Kotzebue, at the Marshall Islands, and he remarked also that 
they often brought stones in their roots. Similar facts have 


THE COMPLETED ATOLL. 317 


been observed at the Gilbert Group, and also at Enderbury’s 
Island, and many other coral islands in the Pacific. The 
stones at the Gilbert Islands, as far as could be learned, are 
generally basaltic or volcanic, and they are highly valued 
among the natives for whetstones, pestles, and hatchets. The 
logs are claimed by the chiefs for canoes. Some of the logs 
seen by the author, like those of Enderbury’s Island, were 
forty feet or more long. Several large masses of compact 
cellular lava occur on Rose Island, a few degrees east of the 
Navigator Group; they were lying two hundred yards inside 
of the line of breakers. The island is uninhabited, and the 
origin of the stones is doubtful; they may have been brought 
there by roots of trees, or perhaps by some canoe. 

Fragments of pumice and resin are transported by the 
waves to many of the islands in the Central Pacific. We 
were informed at the Gilbert Islands that the pumice was 
gathered from the shores by women and pounded up to fer- 
tilize the soil of their taro patches; and that it is common 
for a woman to pick up a peck a day. 

Where this pumice comes from is not ascertained. It is ‘ 
probably drifted from the westward, and perhaps from vol- 
canic islands of the Ladrones or Phillipines. In addition, 
volcanic ashes are sometimes distributed over these islands, 
through the atmosphere. In this manner the soil of the 
Tonga Islands has been improved, and it may thus have de- 
rived its reddish color. This group has its own active vol- 
cano to supply the ashes, and the volcanic group of the 
New Hebrides is not far distant to the southwest. 

The mineralogy of an atoll is usually confined to one 
species, — calcite, or carbonate of lime,— the material of 
the coral rock. On some of the smaller islands, in the drier 
equatorial part of the ocean, there are, in addition to this 


318 CORALS AND CORAL ISLANDS. 


and the stones brought by logs with the floating pumice, beds 
of gypsum which have been made through the evaporation of 
sea-water (which holds it in solution) in the gradually drying 
lagoon-basins; and also large deposits of guano from the 
multitudes of sea birds that occupy them. Such are Jarvis, 
Baker’s, Howland’s, Malden’s, McKean’s, Birnie’s, Starbuck’s, 
Enderbury’s, and probably other islands in the dry central 
Pacific (Plate IX. and p. 168). As these deposits are con- 
nected with the completion of the coral island and its accom- 
panying reduction in size, and illustrate one of the ways by 
which new minerals are added to a destitute land, a few facts 
are here cited from an article in the “ American Journal of 
Science,” volume xxxiv. (1862), by J. D. Hague, who resided 
for several months on the islands he describes. 

Baker’s Island is situated in lat. 0° 13’ north, and long. 
176° 22’ west from Greenwich, and, excepting Howland’s 
Island, forty miles distant, is very remote from any other 
land. It is about one mile long and two thirds of a mile 
wide. The surface is nearly level; the highest point is 
twenty-two feet above the level of the sea, showing some 
evidence of elevation. 

Above the crown of the beach there is a sandy ridge 
which encircles the guano deposit. This marginal ridge is 
about one hundred feet wide on the lee side of the island, 
and is there composed of fine sand and small fragments of 
corals and shells, mixed with considerable guano; on the 
eastern, or windward, side it is much wider, and formed of 
coarser fragments of corals and shells, which, in their arrange- 
ment, present the appearance of successive beach formations. 
Encircled by this ridge lies the guano deposit, occupying the 
central and greater part of the island. The surface of this 
deposit is nearly even, but the hard coral bottom which forms 


THE COMPLETED ATOLL. 319 


its bed has a gradual slope from the borders toward the cen- 
tre, or, perhaps more properly, from northwest to southeast, 
giving the guano a variable depth from six inches at the 
edges to several feet. 

Howland’s Island is situated in lat. 0° 51’ north, and long. 
176° 32’ west from Greenwich. It is about a mile and a half 
long by half a mile wide, containing, above the crown of the 
beach, an area of some four hundred acres. The highest 
point is seventeen feet above the reef, and ten or twelve feet 
above the level of high tide. The general features of the 
island resemble those of Baker’s. 

The main deposit of guano occupies the middle part of 
the island, and stretches, with some interruptions of inter- 
vening sand, nearly from the north to the south end. Its 
surface is even, and in many places covered by a thick 
growth of purslane, whose thread-like roots abound in the 
guano where it grows. The deposit rests on a hard coral 
bottom, and varies in depth from six inches to four feet. 
The fact observed at Baker’s that vegetation flourishes most 
where the guano is shallow, is also quite apparent here, and 
the consequent characteristic difference between the 2uano 
of the deep and shallow parts is distinctly marked. 

Some interesting pseudomorphs occur buried in the guano 
of this island. Coral fragments of various species were found 
that had long been covered up under the deposit, and in some 
of which the carbonic acid had been almost entirely replaced 
by phosphoric acid. In such I have found seventy per cent 
of phosphate of lime. In many others the change was only 
partial, and, on breaking some of these, in the centre was 
usually found a nucleus or core of coral. 

Jarvis Island is situated in lat. 0° 22’ south, and long. 
159° 58’ west from Greenwich. It isnearly two miles long by 


320 CORALS AND CORAL ISLANDS. 


one mile wide, and contains about 1,000 acres. Like Baker’s 
and Howland’s, it has the general features of a coral island, 
but it differs from them essentially in the fact that the 
lagoon, which it once contained, has gradually been filled up 
with sand and detritus, while the whole island has undergone 
some elevation. “It therefore presents a basin-like form, the 
surface being depressed from the outer edge toward the cen- 
tre. It is encircled by a fringing reef, or shore platform, 
about three hundred feet wide; from this a gradually sloping 
beach recedes, the crown of which is from eighteen to twenty- 
eight feet high, forming a ridge or border, of varying width, 
which surrounds the island like a wall, from the im-shore 
edge of which the surface of the island is gently depressed. 

Within this depression there are other ridges, parallel 
to the outer one, and old beach lines and water marks, the 
remaining traces of the waters of the lagoon, marking its 
gradual decrease and final disappearance. 

This flat depressed surface in the centre of the island is 
about seven or eight feet above the level of the sea. It bears 
but little vegetation, consisting of long, coarse grass, Mesem- 
bryanthemum, and Portulaca, and this is near the outer 
edges of the island, where the surface is formed of coral sand, 
mixed with more or less gtano. In the central and lower 
parts the surface is composed of sulphate of lime (gypsum), 
and it is on this foundation that the principal deposit of 
guano rests. 

In examining the foundation of the guano deposit on 
Baker's or Howland’s Island by sinking a shaft vertically, 
the hard conglomerate reef-rock is found directly underlying 
the guano. Resting on this foundation the guano has under- 
gone only such changes as the climate has produced. On 


Jarvis Island, however, after sinking through the guano, 


THE COMPLETED ATOLL. 321 


one first meets with a stratum of sulphate of lime (sometimes 
compact and crystalline, sometimes soft and amorphous) fre- 
quently two feet thick, beneath which are successive strata 
of coral sand and shells, deposited one above the other in the 
gradual process by which the lagoon was filled up. These 
horizontal strata were penetrated to a depth of about twenty 
feet. They were composed chiefly of fine and coarse sand 
with an occasional stratum of coral fragments and shells. 

Of the origin of this sulphate of lime there can hardly be | 
any doubt. As the lagoon was nearly filled up, while, by the 
gradual elevation of the island, the communication between 
the outer ocean and the inner lake was constantly becoming 
less easy, large quantities of sea-water must have been evap- 
orated in the basm. By this means deposits would be formed 
containing common salt, gypsum, and other salts found in the 
waters of the ocean. From these the more soluble parts would 
gradually be washed out again by the occasional rains, leaving 
the less soluble sulphate of lime as we find it here. 

Some additional light is thrown on this matter by the dif- 
ferent parts of the surface, which, though nearly flat, shows 
some slight variety of level. The higher parts, particularly 
around the outer edges, are composed chiefly of coral sand, 
either mixed with or underlying guano. Nearer the centre © 
is a large tract, rather more depressed, forming a shallow 
basin, in which the bulk of the sea-water must have been 
evaporated, the surface of which (now partly covered with 
guano) is a bed of sulphate of lime, while, further, there 1s 
a still lower point, the least elevated of the whole, where the 
lagoon waters were, without doubt, most recently concen- 
trated. This latter locality is a crescent-shaped bed, about 
six hundred feet long by two hundred or three hundred feet 


wide, having a surface very slightly depressed from the outer 
21 


Say CORALS AND CORAL ISLANDS. 


edge toward the middle. Around the borders are incrusta- 
tions of crystallized gypsum and common salt, ripple-marks, 
and similar evidences of the gradually disappearing lake. 
The whole is composed of a crystalline deposit of sulphate 
of lime, which, around the borders, as already observed, is 
mixed with some common salt, while near the centre, where 
rain water sometimes collects after a heavy shower, the salt 
is almost entirely washed out, leaving the gypsum by itself. 
It is closely, but not hard, packed, and is still very wet.- By 
digging eighteen to twenty-four inches down, salt water may 
generally be found. 

These facts help us to understand the varying conditions 
in which we now find the guano beds. The most important 
part, and that from which the importations have thus far 
come, rests on a bed of sulphate of lime, of an earlier but 
similar origin to that just described above; part rests on a 
coral formation; while still another part, covering a large 
tract, has been by the action of water mixed with coral mud. 

The first named deposit, lying on the sulphate of lime 
bed, has a peculiar character. It is covered by, or consists 
of, a hard crust, that is from one-fourth of an inch to an 
inch and a half in thickness, beneath which hes a stratum of 
~ guano, varying in depth from one inch to a foot. In many 
places where the guano was originally shallow, the whole is 
taken up and formed into the hard crust which then lies im- 
mediately on the sulphate. This crust, when pure, is snow- 
white, with an appearance somewhat resembling porcelain, 
but is usually colored more or less by organic matter. Gen- 
erally it is very hard, and strongly cohesive, though some- 
times friable, and it lies unevenly on the surface in rough 
fragments that are warped and curved by the heat of the 
sun. It consists chiefly of phosphoric acid and lime, but, 


THE COMPLETED ATOLL. 323 


owing to the variable amount of sulphate of lime with which 
it is mechanically mixed, there is a lack of uniformity in 
different samples. Hence the percentage of phosphoric acid 
varies from over fifty per cent to less than thirty per cent. 

The gypsum, or sulphate of lime, is usually soft and amor- 
phous, sometimes crystalline, and, at a depth of eighteen 
inches to two feet, occurs in hard, compact, crystalline beds. 
It is of a light snuff color, and where it underlies guano, is 
mixed with considerable phosphate of lime, which has been 
washed down from the surface. Similar deposits of sul- 
phate of lime occur on many other elevated lagoon islands 
of the Pacific. 

Starbuck’s, Starve, or Hero Island is an elevated atoll, and 
is worthy of mention, because like Jarvis, McKean’s, and 
other islands of similar structure, it contains a large deposit 
of gypsum. Its supposed guano I have found to consist of 
the hydrated sulphate of lime, containing about twelve per 
cent of phosphate of lime, and colored by a little organic 
matter. So far as my observation extends, all elevated 
lagoons have similar deposits of gypsum. ae 
PAs regards the distribution of these phosphatic guano 
deposits, I believe them, in this region of the Pacific, to be 
confined to latitudes very near the equator, where rain is 
comparatively of rare occurrence. In latitudes more remote 
from the equator than 4° or 5°, heavy rains are frequent, and 
this circumstance is not only directly unfavorable to the for- 
mation of guano deposits, but it encourages vegetation, and 
when an island is covered with trees and bushes, the birds 
prefer to roost in them, and there is no opportunity for the 
accumulation of guano deposits. 

An article in the same Journal (vol. xl., 1865), by A. A. 
Julien, gives an account of the various phosphatic minerals 


324 CORALS AND CORAL ISLANDS. 


formed from the guano deposits on a coral island, Sombrero, 
in the Caribbean Sea. 

Lord Byron, of the ‘ Blonde,” mentions that phosphate 
of lime (apatite) was collected by him on Mauke, an elevated 
coral island of the Hervey Group, west. of the Society Islands, 
but its exact condition in the rock is not stated. 

Water is to be found commonly in sufficient quantities for 
the use of the natives, although the land is so low and flat. 
They dig wells five to ten feet deep in any part of the dry 
islets, and generally obtain a constant supply. These wells 
are sometimes fenced around with special care; and the 
houses of the villagers, as at Fakaafo, are often clustered 
about them. On Aratica (Carlshoff) there is a. watering 
place fifty feet in diameter, from which vessels of the Wilkes 
Exploring Expedition obtained three hundred and ninety gal- 
lons. The Gilbert Islands are generally provided with a sup- 
ply sufficient for bathing, and each native takes his morning 
bath in fresh water, which is esteemed by them a great lux- 
ury. On Tari-tari (of the Gilbert Group, p. 165), as Mr. 
Horatio Hale, philologist of the same ‘expedition, was in- 
formed by a Scotch sailor by the name of Grey, taken from 
the island, there is a trench or canal several miles long, and 
two feet deep. They have taro plantations (which is possible 
only where there is a large supply of water), and besides some 
bread-fruit. He spoke of the taro as growing to a very large 
size, and as being in great abundance; it was planted along 
each side of the pond. Grey added further that ten ships of 
the line might water there, though the place was not reached 
without some difficulty. There were fish in the pond which 
had been put in while young. The bottom was adhesive like 
clay. These islands have been elevated a little, but are not 


over fifteen feet above the sea. 


THE COMPLETED ATOLL. 325 


Kotzebue observes, that “in the inner part of Otdia (one 
of the Marshall Islands) there is a lake of sweet water; and 
in Tabual, of the Group Aur, a marshy ground exists. There 
is no want of fresh water in the larger islands; it rises in 
abundance in the pits dug for the purpose.” (Voyage, London, 
1821, i. 145.) 

The only source of this water is the rain, which, perco- 
lating through the loose sands, settles upon the hardened 
coral rock that forms the basis of the island. As the soil is 
white or nearly so, it receives heat but slowly, and there is 
consequently but little evaporation of the water that is once 
absorbed. 

Water is sometimes obtained by making a large cavity in 
the body of a cocoanut tree, two feet or so from the ground. 
At the Duke of York’s Island, and probably also at the adja- 
cent Bowditch Island, this method is put in practice; the 
cavities hold five or six gallons of water. 

The extensive reefs about coral islands, as already stated, 
abound in fish, which are easily captured, and the natives, 
with wooden hooks, often bring in larger kinds from the 
deep waters. From such resources a population of seven 
thousand persons is supported on the single island of Tapu- 
teuea, whose whole habitable area does not exceed six square 
miles, 

There are also shell-fish of edible kinds, and others that 
are the source of considerable activity in pearl-fishing. 

Although the vegetation of coral islands has the luxuri- 
ance that characterizes more favored tropical lands, the num- 
ber of species of land plants is small. A work on the 
“Botany of the Paumotus” would contain descriptions of 
only twenty-eight or thirty species. The most common kinds 


are the following : — 


326 CORALS AND CORAL ISLANDS. 


Portulaca oleracea, L. (lutea Cassytha filiformis, Z. 

of Solander). Gouldia Romanzoffiensis, A. Gr. 
Triumfetta procumbens, Forst. Euphorbia Chamissonis, Boiss. 
Tournefortia argentea, L. Boerhavia diffusa, L. 
Scevola Konigii, Vahl. Boerhavia hirsuta, Wild. 
Ipomea longiflora, R. Br. Achyranthes canescens, &. Br. 
Lepidium piscidium, Forst. Heliotropium prostratum, &. Br. 
Pemphis acidula, Lorst. Nesogenes euphrasioides, A. DC. 
Pandanus odoratissimus, L. f. Asplenium Nidus, Z. 
Pisonia grandis, Parkinson. A polypodium, and a species of 
Morinda citrifolia, L. grass. 


Guettarda speciosa, L. 


Still, there is a better supply than might be supposed. 
For the cocoanut, in view of its uses, is a dozen trees in 
one. Its trunk furnishes timber for the houses of the natives, 
and the best of wood, on account of its weight and strength, 
for clubs and spears, — weapons much in use, besides serving 
as ornamental side-arms. Its leaves supply material for 
thatching; for coarse matting to sit on, and beautiful fine 
mats for use in the way of occasional dress; also for the 
short aprons or petticoats of the women, above alluded to. 
The fruit, besides its delicately flavored hollow kernel, af- 
fords, by the grating of this kernel, a milky juice that is 
richer than cream for purposes of native cookery, and which 
we explorers often used with satisfaction in coffee, cows be- 
ing unknown in those regions; also, from each nut, a pint 
of the thinner “cocoanut milk,’ a more agreeable drink in 
the land of cocoanuts than in New York; also an abundant 
oil, much valued for. sleeking down their naked bodies, and 
sometimes offered to a friendly visitor whom they would 
honor with a like anointing. Further, from the young fruit, 
three fourths grown, comes a delightful beverage as brisk 
nearly as soda-water, besides a rich creamy pulp; both of 
these far better than the corresponding products of the ripe | 


THE COMPLETED ATOLL. aan 


fruit. The husk is excellent for cordage, twine, thread, fish- 
ing-lines; and the smaller cord serves in place of nails for 
securing together the beams of their domestic and public 
buildings, and also for ornamenting the structure within, the 
cord being often wound with much taste and diversity of fig- 
ures. The nut, when opened, is a ready-made drinking-cup 
or cooking utensil. Finally, the developing bud, before 
blossoming, yields a large supply of sweet juice, from which 
molasses is sometimes made, and then, by fermentation, a 
spirituous liquor, called among the Gilbert Islanders by a 
name that sounded very much lke toddy, and possessing 
qualities that answer to the name; but this is procured at 
the expense of the fruit, and the good of the tree, and also 
of the best interests of the natives. 

It is doubted whether the ocean is ever successful in plant- 
ing the cocoanut on coral islands. The nut seems to be well 
fitted for marine transportation, through its thick husk, which 
serves both as a float and a protection; but there is no known 
evidence that an island never inhabited has been found sup- 
plied with cocoanut trees. The possibility of a successful 
planting by the waves cannot be denied; but there are so 
many chances that the floating nut will be kept too long in 
the water, or be thrown where it cannot germinate, that the 
probability of a transplanting is exceedingly small. This 
palm — the Cocos nucifera of the botanists —is not included 
in the above list of native Coral Island plants. 

Another tree, peculiarly fitted for the region, is the Pan- 
danus or Screw-Pine—well named as far as the syllable screw 
goes, but having nothing of a pine in its habit. Its long 
sword-like leaves, of the shape and size of those of a large 
Iris, are set spirally on the few awkward branches toward 
the extremity of each, and make a tree strikingly tropical in 


328 CORALS AND CORAL ISLANDS. 


character. It grows sometimes to a height of thirty feet. It 
is well fitted for the poor and shallow soil of a coral island ; 
for, as it enlarges and spreads its branches, one prop after 
another grows out from the trunk and plants itself in the 
ground ; and by this means its base is widened and the grow- 
ing tree supported. The fruit, a large ovoidal mass made up 
of oblong dry seed diverging from a centre, each near two 
cubic inches in size, affords a sweetish husky article of food, 
which, though little better than prepared corn stalks, admits 
of being stored away for use when other things fail; and at 
the Gilbert Islands and others in that part of the ocean it is 
so employed. 

The Pisonia is another of the forest trees, and one of 
handsome foliage and large beautiful flowers, sometimes 
attaining a height of forty feet, and the trunk a girt of 
twenty feet. | 

Among the species that are earliest in taking root in the 
emerging coral debris over the reef, there are the Portulacee 
(species of purslane); the Zriwmphetta procumbens, a creep- 
ing, yellow-flowering plant of the Tilia family; the Tourne- 
fortia sericea, a low, hoary shrub of the family Borraginacee, 
and Scevola Konigii, a sub-fleshy seashore plant. 

On Rose Island, just east of the Navigator Group, Dr. C. 
Pickering, of the Wilkes Exploring Expedition, found only a 
species of Pisonia and of Portulaca. This is a small atoll, 
under water at high-tide, excepting two banks, one of which 
is covered with trees. 

In the Marshall Group, on the contrary, where the vege- 
tation is more varied, and the islands have probably under- 
gone some elevation since they were made, Chamisso observed 
fifty-two species of land plants, and in a few instances the 
banana, taro, and bread-fruit were cultivated. At the ele- 


THE COMPLETED ATOLL. 399 


vated coral island, Metia, north of Tahiti (p. 193), 250 feet 
above the sea, sugar-cane and bread-fruit and many plants 
of the Society Group occur. 

The tropical birds of the islands are often more in keep- 
ing with the beautiful scenery about them than the savage 
inhabitants. On one atoll, — Honden Island, of the Paumo- 
tus, — where no natives had ever dwelt, the birds were so 
innocent of fear, that we took them from the trees as we 
would fruit ; and many a songster lost a tail feather, as it sat 
perched on a branch, apparently unconscious that the world 
contained an enemy. Our ornithologist went ashore with 
powder and shot. But the sportsman could find no pleasure 
in shooting; indeed he could help himself without. 

During a ramble over the island I came across a noble 
bird as white as snow and nearly as large as an albatross. 
In my zeal for science I began to contemplate it as a very 
fine specimen — indeed, a magnificent specimen; and although 
it was not my special line of research, it seemed a failure of 
duty to neglect the opportunity to secure it. By a simple 
process, the work of death is easily accomplished. I went 
up to him. He stood still, not offering to fly. I commenced 
to carry out my plan. A slight point of blood soiled the 
white plumage, and my zeal gave out. It was another's part 
to serve as executioner, not mine; and stroking down his 
feathers and wishing him well, I hurried away. But as I 
glanced back from time to time, on my retreat, there the 
bird stood, his eye still fixed upon me, and that reproachful 
look followed me until a far-off grove came in between us. 
I take it the bird recovered, as I shared not the fate of the 
“Ancient Mariner.” 

Ever since, the words of the old Mariner have seemed 


to rise in melody from that island Paradise : — 


1S) 
sy) 
oS 


CORALS AND CORAL ISLANDS. 


“ He prayeth well who loveth well 
Both man and bird and beast. 
He-prayeth best, who loveth best 

All things both great and small ; 
For the dear God who loveth us 
He made and loveth all.” 


Mr. J. D. Hague gives an account of the birds of Jarvis 
and some other uninhabited islands in the equatorial Pacific, 
in which it appears that, after all, there are evil-doers even 
among tropical birds. He gives the following facts : — 

“From fifteen to twenty varieties of birds may be distin- 
guished among those frequenting the islands, of which the 
principal are gannets and boobies, frigate birds, tropic 
birds, tern, noddies, petrels, and some game birds, as the 
curlew, snipe, and plover. Of Terns there are several spe- 
cies, the most numerously represented of which is what I be- 
lieve to be the Sterna hirundo. These frequent the island 
twice in the year for the purpose of breeding. They rest on 
the ground, making no nests, but selecting tufts of grass, 
where such may be found, under which to lay their eggs. I 
have seen acres of ground thus thickly covered by these birds, 
whose numbers might be told by millions. Between the 
breeding seasons they diminish considerably in numbers, 
though they never entirely desert the island. They are ex- 
pert fishers and venture far out to sea in quest of prey. The 
noddies (Sterna stolida) are also very numerous. They are 
black birds, somewhat larger than pigeons, with much longer 
wings, and are very simple and stupid. They burrow holes 
in the guano, in which they live and raise their young, gen- 
erally inhabiting that part of the deposit which is shallow- 
est and driest. Their numbers seem to be about the same 
throughout the year. The gannet and booby, two closely 
allied species (of the genus Sula), are represented by two or 


THE COMPLETED ATOLL. aol 


three varieties. They are large birds, and great devourers of 
fish, which they take very expertly, not only catching those 
that leap out of the water, but diving beneath the surface for 
them. They are very awkward and unwieldy on land, and 
_may be easily overtaken and captured, if indeed they attempt 
to escape at all on the approach of man. They rest on the 
trees wherever there is opportunity, but in these islands they 
collect in great groups on the ground, where they lay their 
eggs and raise their young. One variety, not very numerous, 
has the habit of building up a pile of twigs and sticks, twenty 
or thirty inches in height, particularly on Howlands, where 
more material of that sort is at hand, on which they make 
their nest. When frightened, these birds disgorge the con- 
tents of their stomachs, the capacity of which is sometimes 
very astonishing. They are gross feeders, and I have often 
seen one disgorge three or four large flying-fish fifteen or 
eighteen inches in length. 

“The frigate bird (Zachypetes aquilus) I have already 
alluded to. It is a large, rapacious bird, the tyrant of the 
feathered community. It lives almost entirely by piracy, 
forcing other birds to contribute to its support. These frig- 
ate birds hover over the island, constantly lying in wait for 
fishing birds returning from the sea, to whom they give chase, 
and the pursued bird escapes only by disgorging its prey, 
which the pursuer very adroitly catches in the air. They 
also prey upon flying fish and others that leap from sea to sea, 
but never dive for fish and rarely even approach the water. 

“The above are the kinds of birds most numerously repre- 
sented, and to which we owe the existing deposits of guano. 
Besides these are the tropic birds, which are found in consid- 
erable numbers on Howland’s Island, but seldom on Jarvis 
or Baker’s. They prefer the former because there are large 


Bye CORALS AND CORAL ISLANDS. 


blocks or fragments of beach rock scattered over the island’s 
surface, under which they burrow out nests for themselves. 
A service is sometimes required of this bird, which may, per- 
haps, be worthy of notice. A setting bird was taken from 
her nest and carried to sea by a vessel just leaving the island. 
On the second day, at sea, a rag, on which was written a mes- 
sage, was attached to the bird’s feet, who returned to the nest, 
bringing with it the intelligence of the departed vessel. This 
experiment succeeded so well that, subsequently, these birds 
were carried from Howland’s to Baker’s Island (forty miles 
distant), and on being liberated there, one after the other, as 
occasion demanded, brought back messages, proving them- 
selves useful in the absence of other means of communica- 
tion. The game birds, snipe, plover, and curlew, frequent 
the islands in the fall and winter, but I never found any evi- 
dence of their breeding there. They do not leave the island 
in quest of prey, but may be seen at low-tide picking up their 
food on the reef which is then almost dry. 

“Some of the social habits of these birds are worthy of 
remark. The gannets and boobies usually crowd together in 
a very exclusive manner. The frigate birds likewise keep 
themselves distinct from other kinds. The tern appropriate 
to themselves a certain portion of the island; each family 
collects in its accustomed roosting-place, but all in peace and 
harmony. The feud between the fishing birds and their 
oppressors, the frigate birds, is active only in the air; if the 
gannet or booby can but reach the land and plant its feet on 
the ground, the pursuer gives up the chase immediately.” 


Notwithstanding the products and the attractions of a 
coral island, it is in its best condition but a miserable place 
for human development, physical, mental, or moral. There 


THE COMPLETED ATOLL. ja5 


is poetry in every feature, but the natives find this a poor 
substitute for the bread-fruit and yams of more favored lands. 
The cocoanut and pandanus are, in general, the only products 
of the vegetable kingdom afforded for their sustenance, and 
fish, shellfish, and crabs from the reefs their only animal 
food. Scanty too is the supply; and infanticide is resorted 
to in self-defence, where but a few years would otherwise 
overstock the half a dozen square miles of which their little 
world consists — a world without rivers, without hills, in the 
midst of salt water, with the most elevated point but ten to 
twenty feet above high tide, and no part more than three 
hundred yards from the ocean. 

In the more isolated coral islands, the language of the 
natives indicates their poverty, as well as the limited produc- 
tions and unvarying features of the land. All words like 
those for mountain, hill, river, and many of the implements 
of their ancestors, as well as the trees and other vegetation 
of the land from which they are derived, are lost to them ; 
and as words are but signs for ideas, they have fallen off in 
general intelligence. To what extent a race of men placed 
in such circumstances is capable of mental improvement, 
would be an interesting inquiry for the philosopher. Perhaps 
the query might be best answered by another, How many 
of the various arts of civilized life could exist in a land 
where shells are the only cutting instruments, — the plants 
of the land in all but twenty-nine in number, — minerals but 
one, — quadrupeds none, with the exception of foreign rats or 
mice, — fresh water barely enough for household purposes, — 
no streams, nor mountains, nor hills? How much of the 
poetry or literature of Europe would be intelligible to per- 
sons whose ideas had expanded only to the limits of a coral 
island; who had never conceived of a surface of land above 


aod CORALS AND CORAL ISLANDS. 


half a mile in breadth, — of a slope higher than a beach, — 
of a change of seasons beyond a variation in the prevalence 
of rains? What elevation in morals should be expected upon 
a contracted islet, so readily over-peopled that threatened 
starvation drives to infanticide, and tends to cultivate the 
extremest selfishness? Assuredly there is not a more un- 
favorable spot for moral or intellectual progress in the wide 
world than the coral island. | 

Still, if well supplied with foreign stores, including a good 
stock of ice, they might become, were they more accessible, a 
pleasant temporary resort for tired workers from civilized 
lands, who wish quiet, perpetual summer air, salt-water bath- 
ing, and boating or yachting; and especially for those who 
could draw inspiration from the mingled beauties of grove, 
lake, ocean, and coral meads and grottoes, where 


“ Life in rare and beautiful forms 
Is sporting amid the bowers of stone.” 





Yet after all, the dry land of an atoll is so limited, its 
features so tame, its supply of fresh water so small, and of 
salt water so large, that whoever should build his cottage on 
one of them would probably be glad, after a short experience, 
to transfer it to an island of larger dimensions, like Tahiti or 
Upolu, one more varied in surface and productions; that has 
its mountains and precipices, its gorges and open valleys, leap- 
ing torrents not less than surging billows, and forests spread- 
ing up the declivities, as well as groves of palms and corals 
by the shores. 


GEOGRAPHICAL DISTRIBUTION. ooo 


CHAPTER IV. 


GEOGRAPHICAL DISTRIBUTION OF CORAL REEFS AND 
ISLANDS. 


THE distribution of coral reefs over the globe depends on 
the following circumstances, arising from the habitudes of 
polyps already explained. 

1. The temperature of the ocean. 

2. The character of coasts as regards (a) the depth of 
water; (b) the nature of the shores; (c) the presence of 
fresh-water streams ; (d) the direction and force of currents. 

3. Liability to exposure to destructive agents, such as 
volcanic heat. 


It has been stated (p. 108) that reef-growing corals will 
flourish in the hottest seas of the equator, and wherever the 
average temperature of the waters of the ocean for the winter 
is not below 68° F. The isothermal line of this temperature 
(or isocryme) forms, therefore, the boundary line of the coral- 
reef seas. Other corals not forming reefs grow in colder seas 
(p. 109) and at great depths, but to those we do not now refer. 

This line traverses the oceans between the parallels 26° 
and 30°, or in general near 28°. But, as has been stated, 
it undergoes in the vicinity of the continents remarkable 
flexures from the influence of oceanic currents, the polar cur- 
rents bending it toward the equator, while the tropical cause 
a divergence. From a comparison of the thermometrical 


336 CORALS AND CORAL ISLANDS. 


observations of various voyages with those of the Expedition, 
the author has been enabled to draw these boundary lines 
with a considerable degree of accuracy, and they are laid 
down on the chart, Plate XVI., from the author’s Exploring 
Expedition Report on Crustacea. 

In the Pacific Ocean, this coral boundary, or isocryme of 
68°, passes near the Galapagos, but to the south of them, 
and thus approaches closely if it does not cross the equator, 
instead of being near the parallel of 28° south, its position 
in mid-ocean. Captain Fitzroy, R.N., found the surface tem- 


. perature of the sea at the Galapagos, from Sept. 16 to Oct. 
2» 18, 1855, 62° to 70° F., while, under the equator, about the 


middle of the Pacific, the range of surface temperature of the 
sea through the year is 81° to 88° F. 

Change of level would make great changes in the boun- 
dary. Mr. A. Agassiz found modern coral limestone near 
Tilibiche, Peru, at an elevation of two to three thousand feet ; 
showing that, in the early part of the coral-reef era, the con- 
tinent on the Pacific side stood this much lower than now, 
and that then corals were growing on part of its Pacific 
border. 

On the side of Asia the boundary line bends far south- 
ward, and reaches the coast of Cochin China within 15° cf 
the equator, although 50° from the equator a little to the 
eastward. On the west side of the Atlantic, the northern line 
starts at Cape Florida, in latitude 15° N., stretches abruptly 
northward, and bends around the Bermudas in latitude 33° N. 
On the African coast opposite, the northern line curves down- 
ward to the latitude of the Cape Verdes, and the southern 
upward nearly to the equator. The following table gives the 
positions of the coral boundary lines where they meet the 
coasts of the continents. 


GHOGRAPHICAL DISTRIBUTION. 3an 


Pacific Ocean. Atlantic Ocean, 
East side of ocean—Northern, Lat. 21° N Thats 7102" Ni. 
Southern. 4° 8, SERS. 
West side of ocean—Northern. 15° N. 26° N. 


Southern. 


30° S., N. Holland. 
29° S., Africa. 


29° §, 

It follows from the above, that while the coral-reef seas 
are about fifty-six degrees wide in mid-ocean, they are 

In the Pacific twenty-five degrees wide on the west coast 
of America, and forty-five degrees on the Asiatic side. 

In the Atlantic about fifteen degrees wide on the African 
coast, and forty-eight degrees on the coast of America. 

If we reckon to the extremity of the bend in the Gulf 
Stream, the whole width of the coral reef sea off the east 
coast of America, will be over sixty-four degrees; while off 
the west coast of America, the width is hardly eighteen degrees. 
It is obvious that these facts enable us to explain many seem- 
ing anomalies in the distribution of coral reefs. 

The other causes which influence the distribution of reefs, 
operate under this more general one of oceanic temperature, 
that is, within the coral-reef boundary lines. ‘The effect of a 
deep abrupt coast on the distribution of reefs has been pointed 
out (p. 114). The unfavorable character of sandy or muddy 
shores, and the action of detritus, marine currents, and fresh 
waters have also been stated (p. 119), and it is not necessary 
to touch again upon these points. 

Not less striking are the effects of volcanic action in pre- 
venting the formation of reefs; and instances of this influ- 
ence are numerous throughout the Pacific. The existence of 
narrow reefs, or their entire absence, may often be thus ac: 
counted for. For example, in the Hawauan Group, the island 
Hawaii, still active with volcanic fires, has but few patches 
of reef about it, while the westernmost islands, which have 


99 


338 CORALS AND CORAL ISLANDS. 


been longest free from such action, have reefs of considerable 
extent. The island of Maui exemplifies well the same gen- 
eral fact. The island consists of two peninsulas: one the 
eastern, recent volcanic in character, with a large crater at 
summit, and the other, the western, presenting every evidence, 
in its gorges and peaks and absence of volcanic cones, of hav- 
ing become extinct ages since. In conformity with the view 
expressed, the coral reefs are confined almost exclusively to the 
latter peninsula. Other examples are afforded by the Samoan 
or Navigator Islands. Savaii abounds in extinct craters and 
lava streams, and much resembles Hawaii in character; it 
bears proof in every part of being the last seat of the vol- 
canic fires of the Samoan Group. Its reefs are consequently 
few and small: there is but a narrow line on part of the 
northern shores, although on the other islands they are very 
extensive. 

This absence of corals results obviously from the destruc- 
tion of the zodphytes by heat, consequent on volcanic action. 
Submarine eruptions, which are frequent as long as a vol- 
cano near the sea is ijn action, heat the waters and destroy 
whatever of life they may contain. After the eruption of Ki- 
lauea, in 1840, there were numerous dead fish thrown on the 
beach; and many such instances in different regions are on 
record. 

The agencies affecting the growth of coral reefs being be- 
fore the mind, we may proceed to notice the actual distribu- 
tion of reefs through the coral seas. The review given is a 
rapid one, as our present object is simply to explain the 
absence or presence of reefs within the coral reef’ limits, by, 
reference to the above facts. 

In the valuable work by Mr. Darwin, the geographical 
distribution of reefs is treated of at length. The facts here 


GHOGRAPHICAL DISTRIBUTION. 339 


detailed have been obtained from independent sources, except 
where otherwise acknowledged. In accounting for the char- 
acter and distribution of reefs, Mr. Darwin appears to attrib- 
ute too much weight to a supposed difference in the change 
of level in different regions, neglecting to allow the requis- , 
ite limiting influence to volcanic agency, and to the other 
causes mentioned. His conclusion that the areas of active 
volcanos in general, are areas of elevation, and not of subsi- 
dence, and the inference that reefs are absent from the shores 
of islands of recent volcanic action on this account, do not 
accord with the facts above stated: for example, the condition 
of Maui, that it has no reefs on the larger half, that of the 
volcanic cone of recent action, but has them on the other 
half whose fires were long since extinct; for it is not prob- 
able that one end has been undergoing elevation, and the 
other subsidence. 

Pacific Ocean.— The west coast of South America is 
known to be without coral reefs even immediately beneath 
the equator ; but the seas of the Galapagos grow some corals. 
The northward deflection of the coral boundary line accounts, 
as has been shown, for their absence. In the Bay of Panama, 
and elsewhere on the coast, north and south, corals occur in 
patches but no reefs, and this is attributed by Verrill to the 
rough tides of ten to twelve feet. Corals are living at La 
Paz, on the Peninsula of California (p. 112). 

In Captain Colnett’s voyage, allusion is made to a beach 
of coral sand on one of the Revillagigedo Islands, in latitude 
18°, and Clipperton Rock is described as an elevated coral 
island. 

Between the South American coast and the Paumotus are 
two rocky islands, Easter or Waihu and Sala-y-Gomez, both 
of which are stated to be without reefs. 


340 CORALS AND CORAL ISLANDS. 


Captain Beechey mentions, however, that at forty-one fath- 
oms, near Sala-y-Gomez, he found a bottom of sand and 
coral. 

The Paumotus commence in longitude 130° W., and em 
brace eighty coral islands, all of which, excepting about eight 
of small size, contain lagoons. Besides these, there are, near 
the southern limits of the archipelago, the Gambier Islands 
and Pitcairn, of volcanic or basaltic constitution. The for- 
mer in 23° §., have extensive reefs. About the latter, in 25° 
S., there are some growing corals, but no proper reefs. 

The Marquesas, in latitude 10° S., have but little coral 
about them; and this is the more remarkable, since they are 
in close proximity to the Paumotus. But their shores are 
mostly very abrupt, with deep waters close to the rocks. An 
island which, before subsidence has commenced, has some ex- 
tent of shallow waters around, might have very bold shores 
after it had half sunk beneath the waves. This would be the 
case with the island of Tahiti; for its mountain declivities are 
in general, singularly precipitous, except at base. The Mar- 
quesas may, therefore, have once had barrier reefs, which were 
sunk from too rapid subsidence; and afterward, on the ces- 
sation of the subsidence, others failed to form again on ac- 
count of the deep waters. 

The Society Islands have extensive coral reefs, with dis- 
tant barriers. The reefs of Tahiti extend, in some parts, a 
mile from the shores. Tetuaroa, to the north of Tahiti, and 
Tubuai, near Bolabola, are lagoon islands. Maitea, east of 
Tahiti, is a sugar loaf truncated at summit, four miles in 
compass, and is said by Forster to have an encircling reef. 

South of the Society Islands, near 25° S., is Rapa, which 
is represented as a collection of rugged peaks without coral 
shores. The Rurutu and Hervey Islands, just northwest of 


GEOGRAPHICAL DISTRIBUTION. 341 


Rapa, have coral reefs fringing the shores. There is no evi- 
dence of recent volcanic action among them. Some of them 
are elevated coral islands, as Mitiaro, Atiu, Mangaia and 
Mauki, and also, according to Stutchbury, Rurutu. Oka- 
tutaia is a low coral island but six or seven feet out of 
water. ‘ 

Between the Paumotus and the longitude of Samoa are 
numerous small islands, all of coral origin. 

The Samoan or Navigator Islands have extensive reefs, 
About Tutuila, owing to its abrupt shores, they are somewhat 
less extensive than around Upolu, and about Savaii they are 
still smaller, as already explained. The influence of abrupt 
shores may also be seen in some parts of Upolu; for example, 
to the west of the harbor of Falifa, where, for several miles, 
there is no reef, except in some indentations of the coast. 
Manua is described as having only shore reefs. 

The Tonga Islands, south of Samoa, for the most part 
abound in coral reefs, and Tongatabu and the Hapai Group 
are solely of coral. Hoa is a moderately high island, with a 
narrow reef. ‘Tafoa an active volcano, and Kao, an extinct 
cone, are without reefs. Vavau, according to Williams (M/s. 
Enterprises, p. 427, Amer. ed.), is an elevated coral island. 
Pylstaarts, near Koa, is a naked rock, with abrupt shores, and 
little or no coral. Sunday Island, farther south (29° 12’S.), 
is beyond the coral-reef limits. 

North of Samoa are the Union group and other islands of 
small size, all of coral. 

/ The Feejee Group, already sufficiently described, abounds 
in reefs of great extent. There are no active volcanoes, 
and, where examined, no evidence of very recent volcanic 
action. The many islands afford a peculiarly favorable re- 
gion for the growth of zodphytes, and the displays of reefs 


342 CORALS AND CORAL ISLANDS. 


and hving corals were the most remarkable seen by the author 
m the Pacific. 

North of the Feejees are numerous islands leading up to 
the Carolines. They are all of coral, excepting Rotuma, 
Horne and Wallis’s Islands, which are high, and have fringing 
or barrier reefs. The reefs of Wallis’s Island are very exten- 
sive. 

The Gilbert or Kingsmill Islands, the Marshall Islands, 
and the Carolines, about eighty in number, are all atolls, ex- 
cepting the three Carolines, Ponape (Pouynipete of Lutke), 
Kusaie (or Ualan), and Truk (or Hogoleu). Between Ponape 
and Ualan, the McAskill Islands, three in number, are of 
coral, but 60 to 100 feet high (Miss. Herald, 1856, p. 193). 

The westernmost of the Sandwich Islands, Kauai and 
Oahu, have fringing reefs, while eastern Maui and the island 
of Hawaii have but few traces of corals. On Hawaii, the 
only spot of reef seen by us, was a submerged patch off the 
southern cape of Hilo Bay. We have already attributed the. 
absence of corals to the volcanic character of the island. The 
small islands to the northwest of Kauai, are represented as 
coral reefs, excepting the rocks Necker and Bird Island; the 
line stretches on to 28° 30’ N., the northern limit of the coral 
seas.  Lisiansky’s Voyage, 1803-6, in the Neva, 4to., Lon- 
don, 1814, pp. 254, 256, contains an account of some of these 
islands; also the Hawaian Spectator, vol. i.; and also a Re- 
port to the U. S. Bureau of Navigation, December, 1867, by 
Capt. Wm. Reynolds, U. S. N., partially reproduced in the 
American Journal of Science for 1871, vol. ii., p. 380. 

The Ladrones, like the Hawaian Group, constitute a line 
or linear series of islands, one end of which has been long 
free from volcanic action, while the other has still its smok- 


ing cones. The appearances of recent igneous action increase 


GEOGRAPHICAL DISTRIBUTION. 343 


therefore as we go northward, and the extent of the coral 
reefs increase as we go southward; no reefs occur about the 
northernmost islands, while they are quite extensive on the 
shores of Guam. This group, consequently, like the Hawai- 
an and Navigator, illustrates the influence of volcanic action 
on the distribution of reefs. 

A short distance southwest of the Ladrones, and nearly 
in the same line, lie extensive reefs. Mackenzie’s is an atoll 
of large size. Yap (or Hap), Hunter’s, Los Matelotas and 
the Pelews (Palao), are high islands, with large reefs. In the 
last mentioned, the reef grounds cover at least six times the 
area occupied by the high land. Still farther south, toward 
New Zealand, lie the large atolls Aiou, Asie and Los Guedes. 

South of the equator again:—The New Hebrides consti- 
tute along group of high islands, remarkable for the absence 
of coral reefs of any extent, though situated between two of 
the most extensive coral regions in the world,—the Feejees 
and New Caledonia. But the volcanic nature of the group, 
and the still active fires of two vents in its opposite extremi- 
ties, are a sufficient reason for this peculiarity. ‘Tanna is one of 
the largest volcanoes of the Pacific; and nearly all the islands 
of the New Hebrides, as far as known, indicate comparatively 
recent igneous action, in which respect they differ decidedly 
from the Feejees. 

The Vanikoro Group, north of the New Hebrides, accord- 
ing to Quoy, has large barrier reefs about the southernmost 
island, Vanikoro; but at the northern extremity of the range 
there is an active volcano, Tinakoro, and no coral. Tikopia, 
to the southeast of Vanikoro, is high and volcanic, according 
to Quoy, though not now with active fires; and it appears from 
the descriptions given, to have no reefs. Mendana, northeast 
of Tinakoro, according to Kruesenstern, as stated by Darwin, 


344 CORALS AND CORAL ISLANDS. 


is low, with large reefs; Duff's Islands have bold summits 
with wide reefs. | 

New Caledonia, and the northeast coast of New Holland, 
with the intermediate seas, constitute one of the grandest reef. 
regions in the world. On the New Caledonia shores (p. 134), 
the reefs are of great width, and occur not only along the 
whole length of the western coast, a distance of 200 miles, but 
extend to the south beyond the main land 50 miles, and north 
150 miles, making in all a line of reef full 400 miles in length. 
Toward the north extremity, however, it is interrupted or bro- 
ken into detached reefs. This surprising extent is partly ex- 
plained by the fact that New Caledonia is not a land of vol- 
canoes; but on the contrary consists of older metamorphic 
rocks. The streams of so large a land might be expected to 
exclude reefs from certain parts: and in accordance with this 
fact, we find the reefs of the windward or rainy side compara- 
tively small, and scarcely indicated on the charts; while on 
the dry or western side, they often extend thirty miles fromthe 
shores. The theory of subsidence accounts fully for the great 
prolongation of the New Caledonia reefs. The reefs indicate 
moreover, the existence of a former land near three times the 
area of the present island. 

Between New Caledonia and the New Hebrides are sev- 
eral high islands, one of which, Lafu, has been described 
(Quart. J. Geol. Soc., 1847, p. 61) by Rev. W. B. Clarke as 
an elevated coral island, with fringing reefs; it appears also 
from the remarks of this writer, that the other islets of what 
is called the Loyalty Group, are of the same kind. Lafu, the 
largest of the number, is about ninety miles in circumference. 

South of New Caledonia lies Norfolk Island, in latitude 
29° S., about which there is said to be some coral, which is 
occasionally thrown on the beach, but no reefs. 


GHOGRAPHICAL DISTRIBUTION. 345 


Between Australia and New Caledonia the islands are all 
of coral. The Australian reef extending south to the east 
cape, in latitude 24° S., has already been described. Such 
long reefs on the shores of continents are not common. In 
the case of Australia, the zodphytes are not exposed to the 
destructive agents usual on continental shores, as the land has 
a dry climate, the shores are mostly rocky, and there are no 
streams of any extent emptying into the ocean. The east cape 
is the southern limit, because here the tropical current, owing 
to the direction of the coast above, trends off to the eastward 
of south, away from the land, while a polar current follows up 
the shores from the south as far as this cape. South of this 
cape there are only a few scattered coral zodphytes. 

The Louisiade Group is, as has been shown, a region of 
extensive reefs. 

The Solomon Islands, as far as ascertained, are but spar- 
ingly fringed, except the two westernmost, which have some 
large frmging and barrier reefs, and include also some atolls. 
They are described by Dr. Guppy in the Transactions and 
Proceedings of the Royal Society of Edinburgh, 1884-86. 
North of the Solomon Islands are some large reef islands. 
New Ireland, according to D’Urville, has distant reefs on 
part of its shores. 

The Admiralty Islands, farther west, are enclosed by bar- 
rier reefs, and beyond this group there are a few lagoon 
islands. 

The north side of New Guinea is mostly without coral. 
There are several islands off this coast, which are conical vol- 
canic summits, and one of them, near New Britain, and an- 
other, Vulcano, near longitude 145° E., are in action. 

From the facts thus far detailed, the connection between 
the prevalence or extent of reefs, and the various causes as- 
signed as limiting or promoting their growth, is obvious. 


346 CORALS AND CORAL ISLANDS. 


The amount of subsidence determines in some cases the dis. 
tance of barrier reefs from shore ; but it by no means accounts 
for the difference in their extent in different parts of a single 
croup of islands. Indeed, if this cause be considered alone, 
every grade of extent, from no subsidence to the largest amount, 
might in many instances be proved as having occurred on a 
single island. Of far greater importance, as has appeared, is 
the voleanic character of the land. At whatever time the ex- 
isting reefs in the Pacific commenced their growth, they be- 
gan about those of the igneous islands whose fires had become 
nearly or quite extinct; and as others in succession were ex: 
tinguished, these became in their turn, the sites of corals, and 
of coral reefs. Those lands whose volcanoes still burn, are 
yet without corals, or there are only limited patches on some 
favored spots. Zodphytes and volcanoes are the land-making 
agents of the Pacific... The latter prepare the way by pour 
ing forth the liquid rock, and building up the lofty summit. 


vu ‘Quiet succeeds, and then commences the work of the zoophyte 


beneath the sea, while verdure covers the exposed heights. © 


“ia _ ¥%& We may add a few more illustrations from other parts of 


the coral-reef seas. 

Along the north and northwest coast of Australia, there 
appears to be little or no coral in the Gulf of Carpentaria, 
while some extensive patches occur on the shores west of this 
Gulf, as far as the northwest cape in latitude 23° S. 

In the East Indies, there are large, scattered reef-islands 
south of Borneo and Celebes, about some of the Molluccas, 
and near the west end of New Guinea. The islands of Timor- 
laut, and Timor, with many of those intermediate, have large 
rects. The Arru Grou» consists wholly of coral. This sea, 
from Arru, to the islands south of Borneo, is more thriving in 
corals than any other in the Kast Indies. 


Y GHOGRAPHICAL DISTRIBUTION. 347 


Another East Indian coral-reef region of some extent, is 
the Sooloo Sea, between Mindanao and the north of Borneo. 
Yet the reefs are mostly submerged. The author saw no wide 
platforms bordering the high lands, like those of the Pacific. 
There are, however, some small coral islets in the Balabac 
Passage. 

In other parts of the East Indies, coral reefs are quite in- 
considerable. Occasional traces, sometimes amounting to a 
fringing reef, occur along Luzon and the other Philippines. 

The Wilkes Exploring Expedition coasted by the west 
shore of Luzon to Manila, and thence by Luban, Mindoro, 
Panay, to Caldera, near Samboangan in Mindanao; and 
through this distance no reefs were distinguished, as would 
have been the case, had there been any of much extent. At 
the last-mentioned place we found coral pebbles on the beach, 
and by dredging, obtained living specimens in six to eight 
fathoms of water. The only large reefs were those between 
Mindoro and the Calaminianes. There are fringing reefs at 
Singapore. The islands of Borneo, Celebes, Java, and Su- 
matra, according to all the authorities seen by the writer, have 
but few coral patches about their shores, although affording 
long lines of coast for their growth. In the China Seas, 
there are numerous shoals, banks, and island reefs of coral. 
Moreover, shore reefs occur about Loochoo, and the islands be- 
tween it and Formosa. But the whole eastern coast of China 
appears to be without coral. Quelpaert’s island, south of Co- 
rea, in 34° N., is described as having coral about it; and this 
has been confirmed by late information. 

Why should the reefs of the Hast India Archipelago be so 
limited in extent, and large parts be almost destitute, notwith- 
standing their situation in the warmest seas of the ocean, and 


in the most favorable region for tropical productions? We are 


348 CORALS AND CORAL ISLANDS. 


not prepared for a full answer to this inquiry; for it would de. 
mand a thorough knowledge of the shores, as well as of the cur- 
rents, and of the former and present condition of volcanic fires. 
From personal observation we may reply satisfactorily, as far as 
regards part of the southern half of the east coast of Su- 
matra. This coast is low, and sandy, or muddy, and thus af: 
fords the most unfavorable place for zodphytes. A strong 
current sweeps through the Straits of Banka, which keeps the 
water muddy, and the shores in constant change. The same 
cause may operate on the coasts of other islands, but we can- 
not say to what extent. 

The East Indies have been remarkable for their volcanoes, 
exceeding, for the area, everv other part of the world; and this 
fact must. have had influence on the formation of coral reefs, 
though there are not data for fixing the extent of the influence. - 
Of the thousand vents which have been in action, several still 
make themselves felt over wide areas. The Sooloo Islands are 
about one hundred in number, and nearly all are pointed with 
voleanic cones, and while some have the broken declivities that 
are marks of age, others have regular slopes, as if but just now 
extinguished; a dozen of these cones may sometimes be seen 
on a single island. These volcanic peaks often rise out of the 
sea, as if their formation had begun with a submarine erup- 
tion. Ina region so extensively and so recently igneous, the 
coral polyps would have found little chance for growth, until 
volcanic action had become comparatively quiet, and deluges 
of hot water ceased. There appears, therefore, to be some 
reason for the fact that the reefs are small, and have seldom 
reached the surface. The Sooloo Sea is but one of the volcan- 
ic centers in these seas. Java, several of the Philippines, and 
other islands south of these last, with the northern shore of 
New Guinea, make up a wide region of fires, and it cannot be 


GEOGRAPHICAL DISTRIBUTION. 349 


doubted that the frequent eruptions prevented the growth of 
any thing more than isolated corals, for a long period, over 
each of these areas. For other causes we must look to the na- 
ture of the coasts, fresh-water streams, and marine currents ; 
we leave it for other investigators to apply the explanation to 
particular coasts. 

The coast of China owes its freedom from corals to the 
cool temperature of the waters, the coast being wholly outside, 
as has been stated, of the coral-reef seas. 

One interesting fact should be noted:—the most extensive 
reefs in the East Indies are to be found in the open seas, be- 
tween the large islands; these islands, at the same time, often 
being without proper reefs, or with mere traces of coral. This 
is the case between Borneoand the range of large islands south ; 
the China Sea is another instance of it; north of New Guinea, 
a few degrees, is another. How far this is due to their being 
distant from the scenes of igneous action, and from the detri- 
tus and fresh water of island streams, remains to be deter- 
mined. A sinking island becomes a more and more favorable 
spot for the growth of coral, as it descends; for as its extent 
diminishes, its streams of fresh water and detritus also de- | 
crease. It might, therefore, be expected, on this account alone, 
that such isolated spots of land, away from all impure waters, 
in the open ocean, should become the bases of large reefs. 
The existence of these reef-islands is, therefore, no necessary 
proof of greater subsidence than the coast adjoining has un- 
dergone. Still the fact of a greater subsidence is not im- 
possible or improbable. 

In the Indian Ocean, the Asiatic coast is mostly free from 
growing coral. The great rivers of the continent are probably 
the most efficient cause of their absence, both directly, through 
their fresh waters, and through the detritus they transport and 


300 GORALS AND CORAL ISLANDS. 

distribute along the shores. It will be observed that this 
agent, so ineffectual on small islands, is one of vast influence 
upon larger lands. Mr. Darwin alludes to small patches of 
coral in the Persian Gulf. Ceylon has some fringing reefs. 

The islands of the Indian Ocean are, to a great extent, 
purely of coral. Of this character are the Laccadives, Mal- 
dives, Keeling’s, the Chagos Group; and, north and east of 
northern Madagascar, Saya de Malha, Amirante, Cosmoledo ; 
and also, nearer northern Madagascar, a series of raised atolls 
from Farquhar Island to Aldabra Island. 

Madagascar has a fringing reef upon its southwestern 
point, according to Mr. Darwin, and on some parts of the 
coast above: also on the north and eastern shores far down as 
latitude 18° S. The Comoro Islands, between Madagascar and 
the continent, have large barrier reefs. 

The eastern coast of Africa has narrow reefs extending 
north with some interruptions from Mozambique, in latitude 
16° S., to a short distance from the equator. Corals also 
abound in the Red Sea, occurring in some parts on both shores, 
though most frequent on the eastern, from Tor, in the Gulf of 
Suez, to Konfodah. This long continental reef may at first be 
deemed a little remarkable, after what has been stated about 
such reefs elsewhere. Yet the surprise is at once set aside by the 
striking fact that this whole coast, from the isthmus of Suez 
south, has no rivers, excepting some inconsiderable streams. It 
affords, therefore, an interesting elucidation of the subject un- 
der consideration, and confirms the view taken to account for 
the absence of reefs from many continental coasts. It is a fact 
almost universal, that where there are large fresh-water 
streams, there are earthy, or sandy shores; and where there 
are no such streams, rocky shores, though not uniformly occur: 


ring, are common. 


GEOGRAPHICAL DISTRIBUTION. 351 


On the African coast there are coral reefs at Port Natal, 
in latitude 30° S. ; and here, owing to the warm currents from 
the tropical regions, the mean winter temperature of the water 
is not below 68° F. 

Passing from the Indian to the Atlantic Ocean, we find 
little or no coral on the west coast of Africa. The islands of 
Cape St. Ann and Sherboro, south of Sierra-Leone, are de- 
scribed as coral by Captain Owen, R. N., in the Journal of 
the Geographical Society (vol. ii, p. 89); but this has been 
since denied. The Island of Ascension, in 7° 56’S.,and 14° 
16’ W., must have been bordered by growing coral, as Quoy 
and Gaymard mention that a bed of coral rock may be seen 
buried beneath streams of lava. Quoy also states that the 
corals which formed these reefs are no longer found alive, and 
adds that volcanic eruptions have probably destroyed them. 
The cold polar currents along the western African coast are the 
cause of the absence of corals from it, to within six or seven de- 
grees of the equator; and these coid waters may at times ex- 
tend still farther north. The same obstacle to the diffusion of 
species eastward, mentioned as occurring in the Pacific—that is, 
westward currents—exists also in the Atlantic. 

On the American shores of the Atlantic, north of the 
equator, there are few reefs, except in the West Indies. The 
waters of the Orinoco and Amazon, and the alluvial shores 
they occasion, exclude corals from that part of the coast. 

In the West Indies, the reefs of Florida (p. 204), Cuba, 
the Bahamas (p. 213), and of many of the eastern islands are 
well known. On the east coast of Florida they continue up 
as far as Cape Florida, in latitude 25° 40’ N.; and reef-corals 
are living off Charleston, S. C., according to Prof. Verrill. 
There are also said to be patches at intervals along the coast 


352 CORALS AND CORAL ISLANDS. 


of Venezuela and Guatemala; but the west shores of the Gulf ~ 


of Mexico, as well as the northern, like West Florida, are 
mostly low, and without reefs ; they are within the influence 
of the Mississippi and other large rivers. Some species of 
reef corals, however, occur in the vicinity of Aspinwall (p. 
ig) 

South of the equator, on the east coast of South America, 
there are reefs at intervals, from the vicinity of Cape St. 
Roque to the Abrolhos shoals in latitude 18°, as described by 
Prof. C. F. Hartt, while reef corals extend south to Cape Frio. 


Descriptions of part of the Abrolhos reefs are given on page ~ 


140. North of the Abrolhos reefs, there are others of coral 
stretching on to Point Carumba; again, off the Bay of Porto 


Securo, and across the Bay of Santa Cruz; in the vicinity of 


Camamt, around Quieppe Island; along the shores of Ita- 
parica Island; and at Bahia and Periperi; then, after an in- 
terruption, off Maceid, in the vicinity of Pernambuco. More- 
over the Roceas, a cluster of reefs in the latitude of Fernando 
do Noronha, are, as Hartt observes, probably of coral. 


It is thus seen that the earth is belted by a coral zone, 
corresponding nearly to the tropics in extent, and that the 
oceans throughout it abound in reefs, wherever congenial sites 
are afforded for their growth. It has also been shown that 
the currents of extra-tropical seas, which flow westward, and 
are interrupted and trended toward the equator by the con- 
tinents, contract the coral seas in width, narrowing them to a 
few degrees on the western coasts of the continents ; while 
the tropical currents flowing eastward, diverge from the equa- 
tor, and cause the belt to widen near the eastern shores. The 
polar currents flow also by the eastern coasts, preventing the 
warmer waters from increasing the width of the coral zone as 


GEOGRAPHICAL DISTRIBUTION. 353 
much as it is contracted on the western coasts. / Moreover, the 
trend of the coast and its capes produce other modifications in 


the direction of the currents, the most of which are apparent 


in the actual distribution of coral reefs. On the shores of /~ 


the continents it is observed that there are few extensive 


reefs, and the coasts on which they occur are those which, , 
owing to the dryness of the climate, have no great rivers to ° 


pour freshwater and detritus into the sea. Thus the influence 
of continental waters and detritus on the distribution of reefs, 
is shown to be very marked. But about the Pacific Islands, 
where streams are small, the same cause has had little effect, 


seldom doing more than modifying somewhat the shores and _ 
bottom of a harbor. It has been further demonstrated that 


in different groups, as the Ladrones, Sandwich Islands, Navi- 


gators, New Hebrides, there is an inverse relation between | 
° ° | 
the extent of reefs and the evidences of recent volcanic ac- | 


tion in the island; and that the largest reefs exist where | 
there is no proof of former igneous action, or where it has 
long ceased. ‘The existence of large reef-islands in open seas 
where the neighboring lands are mostly destitute of coral 
reefs, harmonizes with the conclusions announced, since such 
islands are in general removed from the deleterious influences 


| 


| 


ana} -- 


just mentioned ; yet it is very probable that in many cases 
of this kind the region of the open sea may have undergone 
a subsidence not experienced equally by the lands either side ; 
for the cases in which such seas contain coral islands are 
many. 

The modifications of form and interruptions of reefs, 
arising from abrupt or sloping shores, and tidal or local 
currents, have also been exemplified. 


23 


304 CORALS AND CORAL ISLANDS. 


CHAPTER V. 


ON CHANGES OF LEVEL IN THE PACIFIC OCEAN.1 


I. EVIDENCES OF CHANGE OF LEVEL. 


It has been shown that atolls, and to a large extent other 
coral reefs, are registers of change of level. From the evi- 
dence thus afforded, the bottom of a large part of the Pacific 
Ocean is proved to have undergone great oscillations in recent 
geological time. In this direction, then, we find the grandest 
teachings of coral formations. In treating the subject we 
necessarily bring into connection with it evidences of change 
of level from other sources. The proofs of change of level 
here considered are the following : — 

A. Evidences of elevation. 

1. The existence on coral or other islands of patches of 
coral reef, and deposits of shells and sand from the reefs, 
above the level where they are at present forming. 

The coral reef-rock has been shown occasionally to in- 
crease, by growth of coral, to a height of four to six inches 
above low-tide level when the tide is but three feet, and to 
twice this height with a tide of six feet. It may, therefore, 
be stated as a general fact, that the limit to which coral may 
grow above ordinary low tide is about one sixth the height of 
the tide, though it seldom attains this height. Its existence 


1 The conclusions and nearly all the facts presented in this chapter are from 
the author’s Exploring Expedition Report, in which the subject is illustrated by a 
colored chart of the Pacific Ocean. 


CHANGES OF LEVEL IN CORAL-REEF REGIONS. 355 


on an island at a higher level would be proof of an elevation 
of the land. 

When the tide is three feet, beach accumulations of large 
masses seldom exceed eight feet above high tide, and the finer 
fragments and sand may raise the deposit to ten feet; but 
with a tide of six feet twice this height may be attained. 
With the wind and waves combined, or on prominent points | 
where these agents may act froin opposite directions, such 
accumulations may be fifteen to twenty feet in height, and 
occasionally thirty to forty feet. These latter are drift de- 
posits, finely laminated, generally with a sandy texture, and 
commonly without a distinguishable fragment of coral or 
shell; and in most of these particulars they are distinct from 
reef-rocks. 

2. On islands not coral, the existence of sedimentary de- 
posits, or layers of rolled stones, interstratified among the 
layers of igneous or other rocks constituting the halls. 

B. Evidence of subsidence. 

1. The existence of wide and deep channels between an 
island and any of its coral reefs; or in other words, the exist- 
ence of barrier reefs. 

2. The existence of lagoon islands or atolls. 

3. The existence of submerged atolls. 

4. Deep bay-indentations in the coasts of high islands as 
the terminations of valleys. —The kind of evidence here re- 
ferred to has been fully explained on page 273. It may be 
added that the absence of coves, or deep-valley bays, may be 
evidence that no subsidence has taken place, or only one of 
comparatively small amount. 

C. Probable evidence of subsidence now in progress. 

1. An atoll reef without green islets, or with but few small 
spots of verdure.—The accumulation requisite to keep the 


356 CORALS AND CORAL ISLANDS. 


reef at the surface-level, during a slow subsidence, renders it 
impossible for the reef to rise above the waves and supply 
itself with soil, unless the subsidence is extremely slow, or 
has wholly ceased. 


From the above review of evidences of change of level, it 
appears that where there are no barrier reefs, and only fringing 
reefs, the corals may afford no evidence of subsidence. But 
it does not follow that the existence of only fringing reefs, or 
of no reefs at all, is proof against a subsidence having taken 
place. For we have elsewhere shown that through volcanic 
action, and at times other causes, corals may not have begun 
to grow till a recent period, and, therefore, we learn nothing 
from them as to what may have previously taken place. 
While, therefore, a distant barrier is evidence of change of 
level, we can draw no conclusion either one way or the other, 
from the fact that the reefs are small or wholly wanting, 
until the possible operation of the several causes limiting 
their distribution has been duly considered. 

The influence of volcanoes in preventing the growth of 
zodphytes extends only so far as the submarine action may 
heat the water, and it may, therefore, be confined within a 
few miles of a volcanic island, or to certain parts only of its 
shores. 

There are two epochs of changes in elevation which may 
be here distinguished and separately considered. 1. The sub- 
sidence indicated by atolls and barrier reefs. 2, Hlevations 


during more recent periods. 


SUBSIDENCE IN PACIFIC CORAL REGIONS. 373)/( 


II. SUBSIDENCE INDICATED BY ATOLLS AND BARRIER 
REEFS. 

Looking at atolls as covering buried islands, we observe 
that through the equatorial latitudes such marks of. subsi- 
dence abound from the Kastern Paumotus to the Western 
~ Carolines, a distance of about six thousand geographical miles. 
In the Paumotu Archipelago there are about eighty of these 
atolls. Going westward, a little to the north of west they 
are found to dot the ocean at irregular intervals; and the 
| Kingsmill or Gilbert Group, the Marshall Group, and the 
Carolines comprise seventy-five or eighty atolls. 

If a line be drawn from Pitcairn’s Island, the southern- 
most of the Paumotus, by the Gambier Group, the north of 
the Society Group, the Navigators, and the Solomon Islands 
to the Pelews, it will form nearly a straight boundary, trend- 
ing N. 70° W., running between the atolls on one side and 
the high islands of the Pacific on the other, the former lying 
to the north of the line, and the latter to the south. 

Between this boundary line and the Hawaiian Islands, an 
area nearly two thousand miles wide and six thousand long, 
there are two hundred and four islands, of which only three, 
exclusive of the eight Marquesas, and the Ladrones with Yap, 
Hunter’s, and Los Matelotas in the line of the Ladrones and 
Pelews, are not of coral. These three are Kusaie or Ualan, 
Ponape, and Truk or Hogoleu, all in the Caroline Archipelago. 
South of the same line, within three degrees of it, there is an 
occasional atoll; but beyond this distance, there are none 
excepting the few in the Friendly Group, and one or two in 
the Feejees. ° 

If each coral island scattered over this wide area indicates 
the subsidence of an island, we may believe that subsidence 


358 CORALS AND CORAL ISLANDS. 


was general throughout the area. Moreover, each atoll, could 

we measure the thickness of the coral constituting it, would 
inform us nearly how much subsidence took place where it 
stands; for they are actually so many registers placed over 
the ocean, marking out, not only the site of a buried island, 
but also the depth at which it lies covered. We have not the 
means of applying the evidence ; but there are facts at hand, 
which may give at least comparative results. 

a. We observe, first, that the barrier reefs are, in general, 

evidence of less subsidence than atoll reets (p. 266). Conse- 
quently, the great preponderance of the former just below the 
southern boundary line of the coral island area, and, farther 
south, the entire absence of atolls, while atolls prevail so uni- 
versally north of this line, are evidence of some depression 
just below the line ; of less, farther south ; and of the greatest 
amount, north of the line or over the coral area. 
. b. The subsidence producing an atoll, when continued, 
gradually reduces its size until it finally becomes so small that 
the lagoon is obliterated ; and, consequently, a prevalence of 
these small islands is presumptive evidence of the greater 
subsidence. We observe, in application of this principle, that 
the coral islands about the equator, five or ten degrees south, 
between the Paumotus and the Gilbert Islands, are the small- 
est of the ocean; several of them are without lagoons, and 
some not a mile in diameter. At the same time, in the Pau- 
motus, and among the Gilbert and Marshall Islands, there are 
atolls twenty to fifty miles in length, and rarely one less than 
three miles. It is probable, therefore, that the subsidence 
indicated was greatest at some distance north of the boundary 
line, over the region of small equatorial islands, between the 
meridians of 150° and 180° W. 

c. When, after thus reducing the size of the atoll, the 


SUBSIDENCE IN PACIFIC CORAL REGIONS. 359 


subsidence continues its progress, or when it is too rapid for 
the growing reef, it finally smks the coral island, which, 
therefore, disappears from the ocean. Now, it is a remarkable 
fact that while the islands about the equator above alluded 
to indicate greater subsidence than those farther south, there 
is over a region north of these islands, — that is, between 
them and the Hawaiian Group, —a wide blank of ocean 
without an island, which is nearly twenty degrees in breadth. 
This area lies between the Hawaiian, the Fanning and the 
Marshall Islands, and stretches off, between the first and last 
of these groups, far to the northwest. YT me, 

Is it not, then, a legitimate conclusion that the subsidence,” 
which was least to the south beyond the boundary line and 
increased northward, was still greater or more rapid over this 


open area ; that the subsidence which reduced the size of the 


islands about the equator to mere patches of reef, was further we 


continued, and caused the total disappearance of islands that 
once existed over this part of the ocean ? en 
_ d. That the subsidence gradually diminished southwest- 
wardly from some point of greatest depression situated to the 
northward and eastward, is apparent from the Feejee Group 
alone. Its northeast portion (see chart) consists of immense 
barriers, with only a few points of rock remaining of the 
submerged land; while in the west and southwest there 
are mountain islands of great magnitude. Again, along the 
north side of the Vanikoro Group, Solomon Islands, and 
New Ireland, there are coral atolls, but scarcely one to the 
south. 

In view of this combination of evidence, we are led to 
believe that the subsidence increased from the south to the 
northward or northeastward, and was greatest between the 


Navigator and Hawaiian Islands, near the centre of the area 


360 CORALS AND CORAL ISLANDS. 


destitute of islands, about longitude 170° to 175° W., and 
latitude 8° to 10° N. 

But we may derive some additional knowledge respecting 
this area of subsidence from other facts. 

Hawauan Range.— We observe that the western islands 
in the Hawaiian Range, beyond Bird Island, are atolls, and all 
indicate a large participation in this subsidence. To the east- 
ward in the range, Kauai and Oahu have only fringing reefs, 
yet in some places these reefs are half a mile to three fourths 
in width. They indicate a long period since they began to 
grow, which is borne out by the features of Kauai showing a 
long respite from volcanic action. We detect proof of sub- 
sidence, but not of a large amount. Moreover, there are no 
deep bays; and, besides, Kauai has a gently-slopime coast 
plain of great extent, with a steep shore acclivity of one to 
three hundred feet. The facts favor the idea of much less 
subsidence since the time when the corals began to grow in 
the region of Kauai and Oahu than along the range to the 
westward. The rather small width of the reefs about these 
two islands may be owing to the former action of their vol- 
canoes, which may have been burning during the earlier part 
of the coral-reef era. But deep-sea soundings must be made 
along the whole chain, including the line to the westward, 
before we can speak positively about the change of level. 

The western islands of the range bear some evidence of 
having, in recent times, commenced a new subsidence after a 
temporary cessation. They all have little dry land and vege- 
tation about the reefs. Brooks’s Island, in latitude 28° 15'N., 


— 


and longitude 177° 20’ W., eighteen miles in circumference, 
has on its north and east sides a compact coral wall of about 
five feet elevation, which continues for four and a quarter 


miles, and then becomes a line of detached rocks at tide level. 


SUBSIDENCE IN PACIFIC CORAL REGIONS. 361 


This bare wall, thus described by Capt. Wm. Reynolds, 
U.S. N., appears to be an indication that the land was once 
finished off under a cessation of subsidence, but that a sink- 
ing of small amount has since taken place, amounting per- 
haps to four or five feet. 

Ocean Island, in 28° 25’ N., 178° 25’ W., another of this 
range, is very similar to Brook’s in its wall of coral rock on 
the east; and so also is Pearl and Hermes’ reef, in 27° 50’ 
N., 176° W., though the wall of the latter is more a series of 
detached rocks than a continuous parapet. 

Marquesas.—The Marquesas are remarkable for their ab- 
rupt shores, often inaccessible cliffs, and deep bays. The ab- 
sence of gentle slopes along the shores, their angular features, 
abrupt soundings close alongside the island, and deep inden- 
tations, all bear evidence of subsidence to some extent; for 
their features are very similar to those which Kauai or Ta- 
hiti would present, if buried half its height in the sea, leaving 
only the sharper ridges and peaks out of water. They are 
situated but five degrees north of the Paumotus, where eighty 
islands or more have disappeared, including one at least fifty 
miles in length. There is sufficient evidence that they partici- 
pated in the subsidence of the latter, but not to the same ex- 
tent. They are nearly destitute of coral, and apparently be- 
cause of the depth of water about the islands. 

Gambier Growp.—In the southern limits of the Paumotu 
Archipelago, where, in accordance with the foregoing views, 
the least depression in that region should have taken place, 
there are actually, as we have stated, two high islands, Put- 
cairn’s and Gambier’s. There is evidence, however, in the ex- 
tensive barrier about the Gambier’s (see cut on page 266), 
that this subsidence, although less than farther north, was by 
no means of small amount. On page 157, we have estimated 


362 CORALS AND CORAL ISLANDS. 


it at 1,150 feet—possibly 1,750. These islands, therefore, 
although toward the limits of the subsiding area, were still far 
within it. The valley-bays of the islets of the lagoon are of 
great depth, and afford additional evidence of the subsidence. 

Tahitian Islands.—The Tahitian Islands, along with Sa- 
moa and the Feejees, are near the southern limits of the area 
pointed out. Twenty-five miles to the north of Tahiti, within 
sight from its peaks, lies the coral island Tetuaroa, a register 
of subsidence. Tahiti itself, by its barrier reefs, gives evidence 
of the same kind of change; amounting, however, as we have 
estimated, to a depression of but two hundred and fifty or 
three hundred feet. The northwestern islands of the group 
lie more within the coral area, and correspondingly, they have 
wider reefs and channels, and deep bays, indicating a greater 
amount of subsidence. 

Samoan or Navigator Growp—tThe island of Upolu has 
extensive reefs, which, in many parts are three-fourths of a 
mile wide, but no inner channel. The subsidence is estimated 
on page 158, at one or two hundred feet. The volcanic land 
west of Apia, declines with an unbroken gradual slope of 
one to three degrees beneath the sea. The absence of a low 
cliff is probable evidence of a depression, as has been else- 
where shown. The island of Tutuila has abrupt shores, deep 
bays and little coral. It appears probable, therefore, that it 
has experienced a greater subsidence than Upolu. Yet the 
central part of Upolu has very similar bays on the north, 
which would afford apparently the same evidence; and it is 
quite possible that the facts indicate a sinking which either 
preceded the ejections that now cover the eastern and west- 
ern extremities of Upolu, or accompanied this change of level. 
The large island of Savaii, west of Upolu, has small reefs, 
small because, probably, of volcanic action; for it bears, every- 


SUBSIDENCE IN PACIFIC CORAL REGIONS. 363 


where, evidence of comparatively recent eruption; from it, 
therefore, we gather no certain facts bearing on this subject. 
East of Tutuila is the coral island, Rose. It may be, there- 
fore, that the greatest subsidence in the group was at its 


eastern extremity. 
Feeee Islands. — We have already remarked upon the 


change of level in this group. A large amount of subsidence 
is indicated by the extensive reefs in every portion of the 
group, but it was greatest beyond doubt, in the northeastern 
part. The subsidence, where least, could hardly have been 
less than 2,000 to 5,000 feet. 

We have thus followed around the borders of the coral 
area; and, besides proving the reality of the limits, have 
ascertained some facts with reference to a gradual diminu- 
tion of the subsidence toward, and beyond, these limits. A 
Ine through the Hawaiian Group would pass along the 
northern boundary line of the area; and, taking the southern 
boundary as given on page 357, the oblong area narrows 
eastward. An axis nearly bisecting this space, drawn from 
the eastern Paumotus toward Japan, passes through the re- 
gion of greatest subsidence, as above determined and now 
proved by soundings, and may be considered the line of great- 
est depression for the area of subsidence. 

It is worthy of special note, that this axial line, or line 
of greatest depression, coincides in direction with the mean 
trend of the great ranges of islands, it having the course, N. 
56° W.; and it also corresponds approximately with the 
axial line of the Pacific Ocean. On the map of the Phoenix 
Group, Plate XI, the axial line is drawn just as it was laid 
down in the author’s Expedition Report. Moreover, this map 
gives the soundings that have been recently made in the seas, 
and it is interesting to find full confirmation of the conclu- 








364 CORALS AND CORAL ISLANDS. 


sion that was derived over fifty years since from the size and 
distribution of the atolls; for the line, A A, lies in the 3,000- 
4,000 area of the central Pacific, and, moreover, passes over 
the deepest point in it yet observed, — the sounding giving 
a depth of 3,448 fathoms. The deep central area of the 
Pacific apparently extends, in the direction of this axial line, 
to the still deeper waters east of Japan. 

The southern boundary line of the coral area, as we have 
laid it down, hes within the area of subsidence, although near 
its limits. This area has been prolonged southeastward in 
some places beyond the boundary line. One of the regions 
of this prolongation lies between the Samoan or Navigator 
Group and the Feejees and Tonga Group; another is east of — 
Samoa, along by the Hervey Group. Each of these exten- 
sions trends parallel with the groups of islands. Tt would 
seem, therefore, that the Society and Samoan Islands were 
regions of less change of level than the deep seas either side 
of them; that, therefore, instead of a uniform subsidence over — 
the subsiding area, shading off toward the borders, there were 
troughs of greater subsidence, whose courses were parallel to 
the ranges of islands; that, im other words, there were in the 
ocean's bottom, a few broad synclinal and anticlinal flexures, | 
having a common direction nearly parallel to the axial line 
of the Pacific. The Marquesas and Fanning Groups lie in a 
common line, and thus may mark the course of a great cen- 
tral anticlinal in the oceanic basin. 

The Hawaiian range has probably experienced its greatest 
subsidence to the northwest, where the islands are all atolls, 
and show some evidences of recent sinking; and this north- 
western extremity of the range is nearer to the axis of the area 
of subsidence, above laid down, than is the southwestern. 

What is the extent of the subsidence indicated by the coral 


SUBSIDENCE IN PACIFIC CORAL REGIONS. 365 


reefs and islands of the Pacific? It is very evident that the 
sinking of the Society, Samoan, and Hawaiian Islands has 
been small compared with that required to submerge all the 
lands on which the Paumotus and the other Pacific atolls rest. 
One, two, or five hundred feet, could not have buried the 
many peaks of these islands. Even the 1,200 feet of depres- 
sion at the Gambier Group is shown to be at a distance from 
the axis of the subsiding area. The groups of high islands 
above mentioned contain summits from 4,000 to 14,000 feet 
above the sea; and it is not probable that throughout this 
large area, when the two hundred islands now sunken were 
above the waves, there were none of them equal in altitude 
to the mean of these heights, or 9,000 feet. Hence, however 
moderate our estimate, there must still be allowed a sinking 
of many thousand feet. Moreover, whatever estimate we 
make that is within probable bounds, we shall not arrive at 
a more surprising change of level than the continents show 


that they have undergone; for since the Tertiary began (or , 


the preceding period, the Cretaceous, closed) more than 10,000 


feet have been added to the Rocky Mountains, parts of the ~' 


Andes, and Alps, and 19,000 feet to part of the Himalayas. ~ 

Between the New Hebrides and Australia, the reefs and 
islands mark out another area of depression, which may have 
been simultaneously in progress. The long reef of one hun- 
dred and fifty miles from the north cape of New Caledonia, 
and the wide barrier on the west, cannot be explained with- 
out supposing a subsidence of one or two thousand feet at the 
least. The distant barrier of Australia is proof of great sub- 
sidence, even along the border of that continent. But the 
ereatest amount of sinking took place, in all probability, over 
the intermediate sea, called the “‘ Coral Sea,’”’ where there are 


now a considerable number of atolls. fae 


— 


366 CORALS AND CORAL ISLANDS. 


Il. EFFECT OF THE SUBSIDENCE. 


The facts surveyed give us a long insight into the past, 
and exhibit to us the Pacific once scattered over with lofty 
lands, where now there are only humble monumental atolls. 
Had there been no growing coral, the whole would have 
passed without a record. These permanent registers, exhibit 
in enduring characters some of the oscillations which the 
“stable” earth has since undergone. 

From the actual size of the coral reefs and islands, we 
know that the whole amount of high land lost to the Pacific 
by the subsidence was at the very least fifty thousand square — 
miles. But since atolls are necessarily smaller than the land — 
they cover, and the more so, the further subsidence has pro- 
ceeded ;—since many lands, owing to their abrupt shores, 
or to volcanic agency, must have had no reefs about them, 
and have disappeared without a mark; and since others may 
have subsided too rapidly for the corals to retain themselves 
at the surface; it is obvious that this estimate is far below 
the truth. It is apparent that in many cases, islands now 
disjoined have been once connected, and thus several atolls 
may have been made about the heights of a single subsiding 
land of large size. Such facts show additional error in the 
above estimate, evincing that the scattered atolls and reefs tell 
but a small part of the story. Why is it, also, that the Pa- 
cific islands are confined to the tropics, if not that beyond 
thirty degrees the zodphyte could not plant its growing reeg- 
isters ? 

The island of Ponape, in the Caroline Archipelago, affords 
evidence of a subsidence in progress, as Mr. Horatio Hale, the 
Philologist of the Wilkes Expedition, gathered from a for- 
eizner who had been for a while a resident on this island. 


a 


yr F Mie VN ;* vy iw. F ==, ; 


SUBSIDENCE IN PACIFIC CORAL REGIONS. 367 


Mr. Hale remarks, after explaining the character of certain 
sacred structures of stone: “It seems evident that the con- 
structions at Ualan and Ponape are of the same kind, and 
were built for the same purpose. It is also clear that when 
the latter were raised, the islet on which they stand was 
in a different condition from what it now is. For at present 
they are actually in the water; what were once paths are now 
passages for canoes, and as O’Connell [his informant] says, 
‘when the walls are broken down, the water enters the enclo- 
sures.’” Mr. Hale, hence infers ‘“ that the land, or the whole 
group of Ponape, and perhaps all the neighboring groups, 
have undergone a slight depression.” He also states respect- 
ing a small islet near Ualan, “ From the description given of 
Leilei, a change of level of one or two feet would render it 
uninhabitable, and reduce it, in a short time, to the same state 
as the isle of ruins at Ponape.” 

In some of the northern Carolines, the Pescadores, and 
perhaps some of the Marshall Islands, the proportion of dry 
land is so very small compared with the great extent of the 
atoll, that there is reason to suspect a slow sinking even at the 
present time; and it is a fact of special interest in connection 
with it, that this region is near the axial line of greatest de- 
pression, where, if in any part, the action should be longest 
contanued. 

Among the Kingsmills and Paumotus, there is no reason 
whatever for supposing that a general subsidence is still in 
progress; the changes indicated are of a contrary character. 


IV. PERIOD OF THE SUBSIDENCE. 


The period during which these changes were in progress, 
extends back to the Tertiary era, and perhaps still farther back. 


368 CORALS AND CORAL ISLANDS. 


In the island of Metia, elevated two hundred and fifty feet, 
the corals below were the same as those now existing, as far 
as we could judge from the fossilized specimens. At the in- 
ner margin of shore reefs, there is the same identity with ex- 
isting genera. We do not claim to have examined the base- 
ment of the coral islands, and offer these facts as the only ev- 
idence on this point that is within reach. We cannot know 
with absolute certainty that the present races of zodphytes 
may not be the successors of others that flourished, on the saine 
sites, even before the Tertiary era in Cretaceous and Jurassic 
times; but as yet have little reason in facts observed, for such 


om 


a conclusion. For a long time volcanic action may have been 


too general and constant over the Pacific, for the growth of 
corals; and this may have continued to interfere till a com- 
paratively late period, if we may judge from the appearance 
of the rocks, even on Tahiti. The subsidence has probably tor 
a considerable period ceased in most, if not all, parts of the 
ocean, and subsequent elevations of many islands and groups 


have taken place. 


V. ELEVATIONS OF MODERN ERAS IN THE PACIFIC, 


Since the period of subsidence discussed in the preceding 
pages, there has been no equally general elevation. Yet va- 
rious parts of the ocean bear evidence of changes confined to 
particular islands, or groups of islands. While the former 
exemplify one of the grander events in the earth’s history, in 
which a large segment of the globe was concerned, the latter 
exhibit its minor changes over limited areas. The instances 
of these changes are so numerous and so widely scattered, that 
they afford convincing evidence of a cessation in the previous 


general subsidence. 


ELEVATIONS IN PACIFIC CORAL REGIONS. 369 


ed 


lat. 10° 17 N. and long. 109° 19! W.. is an elevated “atoll at 


least 100 feet, high, according to W. Harper Pease, as re- 


ported in the Proceedings of the California Academy of  ° 


Sciences, Vol. III., p. 199. 

b. Paumotu Archipelago. — The islands of this archipelago 
appear in general to have that height which the ocean may 
give to the materials. Nothing was detected indicating any 
general elevation in progress through the archipelago. The 
large extent of wooded land shows only that the islands have 
been long at their present level. There are examples of ele- 
vation in particular islands, however, some of which are of 


unusual interest. The instances examined by the Expedition 
are those of Honden Island, Dean’s Island, and Clermont 


Tonnerre. Besides these, Elizabeth Island has been described 
by Beechey, and, according to the same author, Ducie’s 
Island and Osnaburgh suggest some elevation. 

Honden Island or Henuake.—This island is wooded on its 
different sides, and has a shallow lagoon. The beach is eight 
feet high, and the land about twelve. There are three entrances 
to the lagoons, all of which were dry at low water, and one 
only was filled at high water. Around the lagoon, near the 
level of high tide, there were numerous deserted shells of the 
huge Tridacna, often a foot long, lying in cavities in the coral 
rock, precisely as they occur alive on the shore reef. As these 
Tridacnas evidently lived where the shells remain, and do not 
occur alive more than six or eight inches, or a foot at the most, 
above low tide, they prove, in connection with the other facts, 
an elevation of at least two feet. 

Nairsa or Dean’s Island—The south side of Dean’s 
Island, the largest of the Paumotus, was coasted along by the 


Peacock, one of the Sloops of War of the Wilkes Exploring 
24 


70) CORALS AND CORAL ISLANDS. 


Expedition, and from the vessel we observed that the rim of 
land consisted for miles of an even wall of coral rock, appar- 
ently six or eight feet above high tide. This wall was broken 
into rude columns, or excavated with arches and caverns; in 
some places the sea had carried it away from fifty to one hun- 
dred rods, and then there followed again a line of columns, 
and walls, with occasional arches as before. The reef, for- 
merly lying at the level of low tide, had been raised above the 
sea, and subsequently had undergone degradation from the 
waves. ‘The standing columns had some resemblance in cer- 
tain parts to the masses seen here and there on the shore plat- 
forms of other islands; but the latter are only distantly scat- 
tered masses, while on this island, for the greater part of the 
course, there were long walls of reef-rock. The height, more- 
over, was greater, and they occurred too on the leeward side 
of the island, ranging along nearly its whole course, while the 
north side, according to the map, 7s wooded throughout. 

The elevation here indicated is at least sia feet; but it 
may have been larger; the observations were made from ship- 
board. 

Thirty miles to the southward of Dean’s Island, we 
come to 

Metia.—This island has already been described, and its el- 
evation stated at two hundred and fifty feet. (See page 193.) 

Clermont Tonnerre shows the same evidence of elevation 
from Tridacnas, as Honden Island. Clermont Tonnerre and 
Honden are on the northeastern limits of the Paumotus. 

Elizabeth Island was early shown to be an elevated coral 
island by Beechey. This distinguished voyager represents 
it as having perpendicular cliffs over fifty feet_in height, 
From his description it is obviously like Metia; the elevation 


ELEVATIONS IN PACIFIC CORAL REGIONS. 371 


is erghty feet. It is one of the southeastern Paumotus, near 
Ducie’s. 

Ducie’s [sland is described by Beechey as twelve feet high, 
which would indicate a probable elevation of one or two feet. 

Osnaburgh Island, according to the same author, affords evi- 
dence of having increased its height since the wreck of the Matil- 
da, in 1792. He contrasts the change from a “reef of rocks,” 
as reported by the crew, to ‘‘a conspicuously wooded island,” 
the condition when he visited it; and states, further, that the 
anchor, iron works, and a large gun (4-pounder) of this vessel 
were two hundred yards inside of the line of breakers. Cap- 
tain Beechey suggests that the coral had grown, and thus in- 
creased the height. But this process might have buried the 
anchor if the reef were covered with growing corals (which 
is improbable), and could not have raised its level. If there 
has been any increase of height (which we do not say is cer- 
tain), it must have arisen from an upheaval. 

ce. Tahitian Group.—tThe island of Tahiti presents no 
conclusive evidence of elevation. The shore plains are said to 
rest on coral, which the mountain débris has covered; but 
they do not appear to indicate a rise of the land. 

The descriptions by different authors of the other islands 
of this group do not give sufficient reason for confidently be- 
lieving that any of them have been elevated. The change, 
however, of the barrier reef around Bolabola into a verdant 
belt encircling the island, may be evidence that a long period 
has elapsed since the subsidence ceased; and, as such a change 
is not common in the Pacific, we may suspect that it has been 
furthered by at least a small amount of elevation. The observa- 
tion by the Rev. D. Tyerman with regard to the shells found at 
Huahine high above the sea, may be proof of elevation; but 
the earlier erroneous conclusions with regard to Tahiti (on 


— 


~—p 


a712 CORALS AND CORAL ISLANDS. 


which island masses of coral are carried by natives up the 
mountain, to leave at the highest point reached, and also to 
mark the limits between the land of different chiefs, and are 
common from these causes, up to a height of fifteen hundred 
feet), teach us to be cautious in admitting it without a more 
particular examination of the deposit. Moreover, shells, even 
large ones, are carried far away from the sea by Hermit 
Crabs (Pagurids). 

d. Hervey and Rurutu. Groups.—These groups lie to the 
southwest and south of Tahiti. 

Mangaia is girted by an elevated coral reef three hundred 
feet in height. Mr. Williams, in his Missionary Enterprises, 
pages 48, 50 and 249, speaks of it as coral, with a small quan- 
tity of fine-grained basalt in the interior of the island; he states 
again that a broad ridge (the reef) girts the hills. 

Atiu (Wateoo of Cook) is a raised coral island. Cook 
Voy., i. 180, 197, observes, that it is “nearly like Mangaia.” 
The land near the sea is only a bank of coral ten or twelve feet 
high, and steep and rugged. The surface of the island is cov- 
ered with verdant hills and plains, with no streams. It is de- 
scribed by Williamsin his Missionary Enterprises. Mauke is 
a low elevated coral island according to Williams, and Mtiaro 
resembles Mauke. Okatutaia is a low coral island, not more 
than six or seven feet high above the beach, which is coral 
sand. It has a light-reddish soil. 

Furutu has an elevated coral reef one hundred and fifty 
feet in height, as stated by Stutchbury, and also Williams. 
Tyerman and Bennet describe the island as having a high cen- 
tral peak with lower eminences, and speak of the coral rock as 
two hundred feet high on one side of the bay and three hun- 
dred on the other (ii, 102).—Ellis says that the rocks of the in- 
terior are in part basaltic, and in part vesicular lava, il. 393. 


| ELEVATIONS IN PACIFIC CORAL REGIONS. 373 


With regard to the other islands of these groups, Manuai, 
Aitutaki, Rarotonga, Rimetara, Tubuar, and Rawavai, the de- 
scriptions by Williams and Ellis appear to show that they have 
undergone no recent elevation. 

é Tonga or Friendly Islands, and others in their vicinity. 

All the islands of the Tonga group about which there are 
reefs, give evidence‘of elevation: Tongatabu and the Hapait 
islands consist solely of coral, and are elevated atolls. 

Hua, at the south extremity of the line, has an undulated 
mostly grassy surface, in some parts eight hundred feet in 
height. Around the shores, as was seen by us from shipboard, 
there is an elevated layer of coral reef-rock, twenty feet thick, 
worn out into caverns, and with many spout-holes. Between 
the southern shores and the highest part of the island, we ob- 
served three distinct terraces. Coral is said to occur at a height 
of three hundred feet. From the appearance of the land, we 
judged that the interior was basaltic ; but nothing positive was 
ascertained with regard to it. 

Tongatabu (an island visited by us) lies near Eua, and is in 
some parts fifty or sixty feet high, though in general but twen- 
ty feet. It has a shallow lagoon, into which there are two en- 
trances ; some hummocks of coral reef-rock stand eight feet out 
of water. 

Namuka and most of the Hapazi cluster, are stated by Cook 
to have abrupt limestone shores, ten to twenty feet in height. 
Namuka has a lagoon or salt lake at centre, one and a half miles 
broad; and there is a coral rock in one part twenty-five feet 
high. It is described by Williams, p. 296. 

Vavau, the northern of the Group, according to Williams 
(p. 427), is a cluster of elevated islands of coral limestone, 
thirty to one hundred feet in height, having precipitous cliffs, 
with many excavations along the coast. 


374 CORALS AND CORAL ISLANDS. 


Pylstaart’s Island, south of Tongatabu, is a small rocky 
islet without coral. Zafua and Proby are volcanic cones, and 
the former is still active. 

Savage Island, a little to the east of the Tonga Group, re- 
sembles Vavau in its coral constitution and cavernous cliffs. It 
is elevated, according to Williams (pp. 275, 276), one hun- 
dred feet. 

Beveridge Reef, a hundred miles southeast of Savage, is low 
coral, 

f. Samoan or Navigator Islands. —No satisfactory evi- 
dences of elevation were detected about these islands. 

g. Atolls, north of Samoa. 

On account of the high tides (four to six feet), the sea 
may give a height of twelve to sixteen feet to the land. 

Swain’s (Gente Hermosas ?), near latitude 11° S., is fifteen 
to eighteen feet above the sea where highest, and the beach 
is ten to twelve feet high. It is a small island, with a de- 
pression at centre, but no lagoon. Probably an elevation of 
two or three feet. This island was named Swain, by Capt. 
Hudson, of the Wilkes Expedition, because not in the position 
assigned to Gente Hermosas. 

Fakaafo, ninety miles to the north, is fifteen feet high. The 
coral reef-rock is raised in some places three feet above the 
present level of the platform. Elevation at least three feet. 

Nukunono, or Duke of Clarence, near Fakaafo, was seen 
only from shipboard. 

Oatafu, or Duke of York’s, is in some parts fourteen feet 
high. Whether elevated or not is uncertain; probably as 
much so as Fakaafo. 

h. Scattered islands farther north, near the equator, east of 
the Gilbert Group. 

Of the Fanning Group, Washington Island, in lat. 4° 41'S., 


ELEVATIONS IN PACIFIC CORAL REGIONS. 375 


and long. 160° 15’ W., is three miles in diameter, and is with- 
out a proper lagoon; the whole surface is densely covered 
with cocoanut and other trees. The height of the land is ten 
or twelve feet. The unusual size of the island for one without 
a lagoon, and the luxuriance of the forest vegetation, are prob- 
able evidence of some elevation, but not beyond three feet. 

Palmyra Island, northeast of Washington, is described by 
Fanning as having two lagoons, the westernmost with twenty 
fathoms water. | 
~ Fanning’s Island, southeast of Washington, according to 
the same voyager, is lower than that island. The accounts 
give no evidence of elevation in either Fanning’s or Palmyra. 

Christmas Island, in lat. 1° 53’ N., long. 157° 32’ W., is 
thirty miles long. Cook speaks of the land as in some parts 
three miles wide, and as having narrow ridges lying parallel 
with the seacoast, which “ must have been thrown up by the 
sea, though it does not reach within a mile of some of 
these places.” The amount of elevation is uncertain. The 
account of J. D. Bennett (Geogr. Journ., vii. 226), represents 
it as a low coral island. 

Jarvis’s Island, in 0° 22’ S., and 159° 58’ W., is, ac- 
cording to J. D. Hague, eighteen to twenty-eight feet in 
height, which would indicate an elevation of at least eight 
or ten feet. See further page 291. 

Malden’s, in 4° 15’ 8., 155° W., two hundred and fifty 
miles southeast of Jarvis, visited by Lord Byron, is described 
by him as not over forty feet high. It is ten miles long. 

Starbuck's, or Hero Island, in 5° 40/ S., 155° 55’ W., is 
an elevated lagoon island; but the amount of elevation is not 
stated. Like Jarvis’s, it contains a large deposit of gypsum, 
but not much guano.—(J. D. Hague.) 

Penrhyn’s Island, near 9° 8. and 157° W., has a length 
of nine miles, and an extensive lagoon with a boat entrance 


376 CORALS AND CORAL ISLANDS. 


into it. According to Captain Ringgold of the Wilkes Ex- 
pedition, it has a height of fifty feet, which, if correct, would 
indicate an elevation of full thirty-five feet. The northwest 
side is, throughout, a cocoanut grove. 

Fint’s Island, in 11° 26’ S., and 151° 48’ W., is only a 
mile and a half long, but is thickly wooded, according to Cap: ‘ 
tain Ringgold, which is unusual for so small an island. 

Staver’s Island, in 10° 05’ 8., and 152° 223’ W., is only 
half a mile across, and yet is well wooded. Both of these 
islands were passed by Captain Ringgold, but he does not 
state the height.—(Wilkes’s Narr., iv. 277.) 

Baker's Island, 0° 13’ N., 176° 22’ W., is one mile 
long and two-thirds of a mile wide. The greatest height, 
according to J. D. Hague, is twenty-two feet, “showing some 
evidences of elevation.” (See further, p. 289.) It has prob- 
ably been elevated at least s¢x feet. 

Howland’s Island, 0° 51’ N., and 176° 32’ W., and about 
forty miles north of Baker’s. It is about one and one-half 
miles long, and one-half mile wide. The highest point, accord- 
ing to Hague, is ten or twelve feet above high-tide level; 
which is evidence of but little if any elevation. It is a guano 
island like Baker’s. 

McKean’s Island, of the Phenix Group (like Phenix, En- 
derbury, and Birnie’s), in 38° 35’S., 174° 17’ W,, is a low 
island, according to Hague, circular in form, one-quarter of a 
mile in diameter, but less elevated than Jarvis Island, It has 
a lagoon depression in which there is a gypsum and guano de- 
posit; and at high tides the guano is sometimes two feet under 
water. Phenia’s Island, near McKean’s, 3° 40’ 8.,170° 52’ 
W., is less than half a mile in diameter, and the border is only 
eight or ten feet high; so that there is no evidence in the 
heicht of an elevation. It is also a guano island. 


ELEVATIONS IN CORAL PACIFIC REGIONS. ate 


Enderbury’s, in 3° 8’ 8., 174° 14’ W., is eighteen feet 
high. It has probably experienced some elevation. But the 
height of the tides is such in the seas as to give the beach and 
drift sands much greater height than they have in the Paumo- 
tus. Birnie’s Island is a small bank of coral, only six feet 
above the sea, according to Wilkes (Narr., V. 4). 

Gardner's, Hulls, Sydney and Newmarket were visited 
by the Wilkes Expedition. No satisfactory evidences of ele- 
vation were observed on the first three. Newmarket is stated 
by Captain Wilkes to have a height of twenty-five feet, which 
would indicate an elevation of six or eight feet. 

h. Sandwich or Hawaian Islands.—Oahu affords decisive 
proof of an elevation of twenty-five or thirty feet. There is an 
impression at Honolulu, derived from a supposed increasing 
height in the reef off the harbor, that the island is slowly ris- 
ing. Upon this point we have nothing satisfactory. The pres- 
ent height of the reef is not sufficiently above the level to which 
it might be raised by the tides, to render it certain, from this. 
kind of evidence, that the suspected elevation is in progress. 

Kauaz presents us with no evidence that the island, at the 
present time, is at a higher level than when the coral reefs be- 
gun ; or, at the most, no elevation is indicated beyond a foot or 
two. The drift sand-rock of Koloa appears to be a proof of 
elevation, from its resemblance to that of Northern Oahu; but 
if so, there must have been a subsidence since, as it now forms 
a cliff on the shore that is gradually wearing away. 

Molokai, according to information from the Rev. Mr. An- 
drews, has coral upon its declivities three hundred feet above 
ie sea, We, Karbon 

Mr. Andrews, in his communication, informed the author 
that the coral occurs ‘“‘ upon the acclivity of the eastern or high- 
est part of the island, over a surface of more than twenty or 


378 


OO 


CORALS AND CORAL ISLANDS. 


thirty acres, and extends almost to the sea. We had no means 
of accurately measuring the height ; but the specimens were ob- 
tained at least three hundred feet above the level of the sea, 
and probably four hundred. The specimens have distinctly the 
structure of coral. The distance from the sea was two to three 
miles.” 

Coral has been reported to occur on the western peninsula 
of Maui, in some places eight hundred feet above the sea; but 
according to C. I. Winslow, the supposed coral does not effer- 
vesce with acids, and therefore is not calcareous. 

On page 324, it is suggested that the westernmost coral 
islands of the Hawaian range, Ocean and Brooks's Islands, 
may have undergone a small subsidence. Should the bro- 
ken wall of emerged rock turn out, on examination, to be 
coral reef-rock, instead of the beach sand-rock, the facts would 
prove an elevation of a few feet, instead of a subsidence. The 
islands differ from Dean’s, in having no long range of wooded 
land on the windward side. 

7. Heejee Lslands.—The proofs of an elevation of four to six 
feet about the larger Feejee Islands, Viti Lebu and Vanua 
Lebu, and also Ovalau, are given in the author’s report on this 
group. How far this rise affected other parts of the group, he 
was unable definitely to determine ; but as the extensive bar- 
rier reefs in the eastern part of the group, rarely support a green 
islet, they rather indicate a subsidence in those parts than an 
elevation. 

j. Islands north of the Feejees.—Horne Island, Wallis, Er 
lice, Depeyster, and four islands on the track toward the 
Kingsmills, were passed by the sloop of war ‘“ Peacock,” of the 
Wilkes Expedition ; but from the vessel no evidences of ele- 
vation could be distinguished. The first two are high islands, 
with barriers, and the others are low coral. Rotuma (177° 








ELEVATIONS IN PACIFIC CORAL REGIONS. 379 


15’ E., and 12° 30’ N.), is another high island, to the west of 

Wallis’s. It has encircling reefs, but we know nothing as to 
its changes of level. According to J. 8. Whitnell, Eilice’s 
Island, or Funafuti, situated in latitude 9° 8. and longitude 
179° E., has a small lagoon basin, dry at low water, which 
is shut off from the sea by a wall twenty feet high, consist- 
ing of large masses of coral. He regarded the facts as 
proof of some elevation. 

k. Kingsmill or Gilbert Group. (Map, p. 165.) 

Tapateuea or Drummond.—This is one of the southern 
islands of the group. The reef-rock, near the village of Utiroa, 
is a foot above low-tide level, and consists of large massive As- 
treas and Meandrinas. The tide in the Kingsmill seas is seven 
feet ; and consequently this evidence of a rise might be doubted, 
as some corals may grow to this height where the tide is so high. 
But these Astreas and Meandrinas, as far as observed by the 
writer, are not among the species that may undergo exposure 
at low tide, except it be to the amount of three or four inches ; 
and it is probable that an elevation of at least one foot has 
taken place. 

Apaiang or Charlotte's Island, one of the northernmost of 
the group, has the re¢f-rock in some parts raised bodily toa height 
of six or seven feet above low-water level, evidencing this 
amount of elevation. This elevated reef was observed for long 
distances between the several’ wooded islets ; it resembled the 
south reef of Nairsa in the Paumotu Archipelago in its bare, 
even top, and bluff, worn front. Anislet of the atoll, where we 
landed, was twelve feet high, and the coral reef*rock was five 
or six feet above middle tide. A wall of this rock, having the 
same height extends along the reef from the islet. ‘There was 
no doubt that it was due to an actual uplifting of the reef to a 
height of full scx feet. 


380 CORALS AND CORAL ISLANDS. 


Nonouti, Kuria, Marana, and Tarawa, lying between the 
two islands above mentioned, were seen only from the ship, and 
nothing decisive bearing on the subject of elevation was ob- 
served. On the northeast side of Nononti there was a hill 
twenty or thirty feet in height covered with trees; but we had 
no means of learning that it was not artificial. We were, how- 
ever, informed by Kirby, a sailor taken from Kuria, that the 
reef of Apamama was elevated precisely like that of Apaiang, to 
a height of fiwe feet ; and this was confirmed by Lieutenant De 
Haven, who was engaged in the survey of the reef. We were 
told, also, that Kuria and Nononti were similar in having the 
reef elevated, though to a less extent. It would hence appear 
that the elevations in the group increase to the northward. 

Marakei, to the north of Apaiang, is wooded throughout. 
We sailed around it without landing, and can only say that it 
has probably been uplifted like the islands south. Makin, the 
northernmost island, presented in the distant view no certain 
evidence of elevation. 

The elevation of the Kingsmills accounts for the long con- 
tinuity of the wooded lines of land, an unusual fact considering 
the size of the islands. The amount of fresh water obtained 
from springs is also uncommon. (p. 324). 

1. The Marshall and Caroline Islands.—The facts in 
reference to the islands of these groups, are not yet fully known. 
The very small amount of wooded land on the Pescadores in- 


clines us to suspect rather a subsidence than an elevation ; and. 


the same fact might be gathered, with regard to some of the 
islands south, from the charts of Kotzebue and Kruesenstern. 
But McAskill’s, as stated on page 342, is an elevated coral 
island, having a height of 100 feet. 

m. Ladrones.—The seventeen islands which constitute this 
group may all have undergone elevations within a recent pe- 


ELEVATIONS IN CORAL PACIFIC REGIONS. 381 


riod, but owing to the absence of coral from the northern, we 
have evidence only with regard to the more southern, 

Guam, according to Quoy and Gaymard, has coral rock upon 
its hills more than six hundred feet (one hundred toises) above 
the sea. 

Rota, the next island north, afforded these authors similar 
facts, indicating the same amount of elevation. 

n. Pelews, and neaghboring Islands.—The island Feis, three 
hundred miles southwest of Guam, of the Ladrone Islands, is. 
stated by Darwin, on the authority of Lutke, to be of coral, 
and ninety feet high. Mackenzie Island, seventy-five miles 
south of Feis, is a low atoll, as ascertained by the Wilkes 
Expedition. In the Pelews elevated reefs occur at various 
heights up to 500 feet, according to Prof. Semper. The most 
of the islands of the southern half of the group are coral 
islands. and are more or less elevated. In Pelelew, the west- 
ern coral cliffs are 250 feet high; the eastern 80 feet. 

0. Melanesian Islands. — 

New Hebrides. — ‘“‘ Much coral at a great altitude,” ac- 
cording to G. Bennett, as reported by Darwin. 

Loyalty Islands. — One of the islands is wholly of coral, 
and is elevated 250 feet, according to Rev. W. B. Clarke, of 
Sydney, in the Journal of the Geological Society, 1847, p. 61. 

Bonin Group. — Peel Island has coral reefs raised 50 feet 
above tide-level, according to P. W. Graves, in the Journal of 
the Geological Society, 1855, p. 552. 

Solomon Islands. — According to Dr. Guppy, there are 
elevated coral reefs, varying in height from 100 to 1,200 feet. 
On St. Christoval, coral occurs to a height not exceeding 500 
feet. 


The details given on the preceding pages are here pre- 


sented in a tabular form. 


382 


Off North American coast . 
Paumotu Archipelago . 


Tahitian Group 


6c ce 


Hervey and Rurutu Groups 


Savage Island. . 


Samoan or Navigator Islands . 


North of Samoa 


Scattered Equatorial Islands 


“e (74 ‘74 


Feejee Islands .. . 


North of Feejees . 


CORALS AND CORAL ISLANDS. 


Clipperton Rock : : 
Bi Gridert syn es Vai hake aie noe a 2 or 3 
Glermont Lonnerre | 2779). 2 or 3 
INairsa or” Dean's) f.:6 a owe ape 6 
Elizabeth . 80 
Metia or Aurora 250 
Ducie’s 1 or 2% 
Tahiti . GE 5 amt ty SG Ae 
Bolabolat. “cir. ai ee eee 2 
Atiu 12% 
Mauke . somewhat elevated. 
Ma tisdto i.e aoe ea ee a & 
Mangaia . 300 
Rurutu 150 
Remaining Islands 0? 
Kua 300? 
Tongatabu : ; 50 to 60 
Namuka and the Hapaii ; 25 
Vavau . 100 
100 
0 
Swain’s Peet i 2 or 3 
Fakaafo, or Bowditch, a Fela he 5) 
Oatafu, or Duke of York’s 2 or 3 
Washington . 2 or 32 
Christmas: 2.4 59 "tc Woe oa ae ? 
Jarvis . . 8 or 10 
Malden’s . (a) er, Deas 20 SOE 
Starbuck’ sic oc) (vee aun et ee ? 
Penrhyn’s : 30 
Flint’sand Stavers” os. eee ? 
Baker's. “2.45 .2hi Shee ee ee 
lowlands). 4 a ee ae MC ? 
Phoenix and McKean’s Si. et ce 0 
Enderbury’s. .. \2 =... 5. =270nnam 
Newmarket 6 or 8? 
Gardner’s, Hull’s, Beir Birnie’ S, 0? 
Viti Levu and Vanua Levu, Ovalau, 5 or 6 
Eastern of the Feejee Islands . 0? 
Horne, Wallis, Depeyster 0? 
or 6 


Ge sere Mee CS: Pech ins OP Ae ee ae ae 


ELEVATIONS IN PACIFIC CORAL REGIONS. ~ 


383 


FEET, 


Sandwich Islands. Kauai . 1 or 2 
66 2 Oahu . 25 to 60 

‘“ é“ Molokai . La 300 ? 
“ 66 DU GUT, Fat I eae a 12 
Gilbert Islands Tapateuea se 3 2 or 3 

6 “6 Nonouti, Kuria, Maiana and 

Tarawa . 3 or more. 
‘ é PAPAMMAMIBy eh lites k ot sol wonots ce 9) 
é «“ Apaiang or Charlotte 6 or 7 
‘ ‘“ Marakei . . 3 or more. 

‘“ «“ THOTT PANE By g Salear seed come Ua ? 
Carolines IMO Naki si ihae feted sol ot teen cles 60 
Ladrones Guam . 600 
«“ Rota 600 
Feis . Sie 90 
Pelews . . 10 to 300 
New Hebrides ... . several elevations. 
Loyalty Islands, one island s 250 
Solomon Islands, some islands - 100 to 1,200 
Bonin Group, Peel Island 40-50 


Several deductions are at once obvious : — 

1. That the elevations have taken place in all parts of 
the ocean. 

2. That they have in some instances affected single islands, 
and not those adjoining. Metia is 250 feet high, and yet the 
other Paumotus in that part of the archipelago, and also the 
Tahitian Islands, have been but little, or not at all, elevated. 

3. That the amount is often very unequal in adjacent 
islands. 

4. That in a few instances the change has been experienced 
by a whole group or chain of islands. The Gilbert Group is 
an instance, and the rise appears to increase from the south- 
ernmost island to Apaiang, and then to diminish again to the 


other extremity. 
The Feejees may be an example of a rise at the west side 


384 CORALS AND CORAL ISLANDS. 


of a group, and possibly a subsidence on the east; while a 
little farther east, the Tonga Islands constitute another ex- 
tended area of elevation. We observe that while the Samoan 
Islands afford no evidences of elevation, the Tonga Islands on 
the south have been raised, and also the Fakaafo Group and 
others on the north. 

We cannot, therefore, distinguish any evidence that a 
general rise is, or has been, in progress ; yet some large areas 
appear to have been simultaneously affected, although the 
action has generally been isolated. Metia and Elizabeth 
Island, in the Paumotu Archipelago, may have risen ab- 
ruptly ;. but the changes of level in the Feejees and the 
Friendly Islands appear to have taken place by gradual 


action. 
] Rat 7 ae BS Ct a 














GEOLOGICAL CONCLUSIONS. 385 


CHAPTER VI. 
GEOLOGICAL CONCLUSIONS. 


THE geological bearing of the facts that have been detailed 
in the preceding pages may have been already perceived by 
our readers. A brief review of the points of more special in- 
terest may serve as a convenient recapitulation of the subject. 


I. FORMATION OF LIMESTONES. 


Coral reefs are beds of limestone made of corals, with the 
help of shells and other calcareous relics of the life of the 
sea. The mode of formation is essentially the same, which- 
ever of the two kinds of organic products — corals or shells 
— predominate ; although in one case the bed would be called 
coral limestone, and in the other, shell limestone. 

The reefs illustrate two different modes of origin of such 
beds: (1), by undisturbed growth, with only additions of fine 
material to fill up the intervals; (2), by the grinding of the 
corals, etc., to fragments, sand, or mud, through the agency 
of the waves. 

Beds made by the former method, have many open spaces 
between the grouped masses or branches, and could not be 
turned into a solid layer of limestone, if situated too deep in 
the ocean to feel sensibly the movement of the waves,—unless 
Rhizopods, or minute shells of some kinds, multiplied so rap- 
idly over the same sea-bottom, as to fill up the interstices. 
There is no reason to believe that such aid from shells or 


86 CORALS AND CORAL ISLANDS. 


Od 


Rhizopods is consistent with the grouping of living corals . 


thickly enough to form reefs. 

The other kind of limestone beds referred to, where un- 
mixed with the former, grow up in compact layers to the 
surface, as a necessary consequence of wave-action ; and lime- 
stones are made in such regions, instead of sandstones and 
shales, because the material exposed to degradation is corals 
and shells, instead of common rocks. 

The facts show that there are formed about coral reefs, 
in indefinite amount, all the ordinary products of degrada- 
tion by wave-action—fragments large and small, down to 
sand, and even mud. With such an agent as the ocean’s 
waves, driven often by the storm, so powerful and so per- 
sistent at lifting, rending, 
little account, at least about outer reefs, that some coral 


grinding, and transporting, it is of 


stems or masses are first weakened below by the boring 
sponge or mollusk; and neither fish, nor holothurian, nor alcy- 
onoid is needed, in order to keep up the supply of particles 
for sand or mud-beds. In accordance with these facts, the reef- 
formations illustrate that not only coral conglomerates, or coral 
rag, may be made of corals, but also the very finest and most 
compact unfossiliferous limestones; that fine compact lime- 
stone, as flint-like in fracture as any of Silurian time, is one of 
the most common of coral-reef rocks, and is nothing but con- 
solidated mud, or fine sand, of coral origin. 

The elevated portion of the island of Metia, which con- 
sists largely of this kind of white, compact, coral-made lime- 
stone, appears to correspond to the interior of the original la 


goon of the island; it exemplifies the kind of rock-making © 


which is going forward in most coral island lagoons. In 
archipelagos like that of the Feejees, where the reef chan- 
nels are very broad, there is an opportunity for the formation 











GHOLOGICAL CONCLUSIONS. 387 


of very large areas of this compact white limestone, and also 
for others of impure or argillaceous limestones. 

Besides the kinds of coral rocks above mentioned, there are 
also the Beach and Drift Sand-rocks, which are accumulated 
and consolidated above low-tide level. These formations illus- 
trate the common mode of origin of odlitéc limestones. 
They also afford numerous examples of the formation of 
coarse and fine conglomerates consisting of beach pebbles— 
these pebbles being either worn corals, or shells, or sometimes 
of other kinds, if other rocks are at hand. 

The general slope of the beach sand-rock and odlite, and 
the mixed stratification of the drift sand-rock, are identical re- 
spectively with those of beach and drift-sand deposits in other 
regions. 


II. BEDS OF LIMESTONE WITH LIVING MARGINS. 


The coral reef as it lies at the water’s level is in fact a bed 
of limestone with living margins; and the living part fur- 
nishes material for its horizontal extension outward, and also, 
if a slow subsidence is in progress, for its increase upward. 
It illustrates an ordinary mode of formation of coral, or of 
shell, limestone, whatever the age. 


Il. MAKING OF THICK STRATA OF LIMESTONE. 


The coral reef-rock has been shown to have in some cases 
a thickness of at least 2,000 feet (page 156.) The reefs are, 
therefore, examples of great limestone strata, nearly as re- 
markable in this respect as the largest of ancient times. 


IV. SUBSIDENCE ESSENTIAL TO THE MAKING OF THICK STRATA. 


The coral island reef-rock has been shown to depend for 
its thickness on a slowly progressing subsidence (p. 263). 


388 CORALS AND CORAL ISLANDS. 


This is the only method by which any thick stratum of lime. 
stone could be made out of a single set of species, for all 
such species have a narrow range in depth ; and the only way, 
from any succession of species, if those species are alike in 
range of depth. | 

In the case of existing coral reefs, there is yet no evidence 
that the species of the lower beds differ from those of the top. 
There is also no evidence, in any part of any ocean, that there 
is a set of cold-water corals fitted to commence a reef in deep 
water and build it up to such a level that another set of 
species may take it and carry it up higher; the facts thus far 
gathered are all opposed to such an idea. Should it be here- 
after proved that the corals of the inferior beds differ in 
species from those now existing, it will probably be found that 
the predecessors of those now living were also shallow-water 
species; so that the subsidence in any case was necessary. 


_V. DEEP-SEA LIMESTONES SELDOM IF EVER MADE FROM CORAL 
ISLAND OR REEF DEBRIS. 
This point has been discussed on pages 143, 211. The facts 
show that the sediment or débris from a shore is almost wholly 
thrown back by the waves against the land where it originated, 


or over its submerged part in the shallow waters, and that it — 


is not transported away to make deep sea formations. 

The facts have also a wider bearing, for they teach that 
lands separated by a range of deep ocean cannot supply one 
another with material for rocks. The existence of an Atlan- 
tic ocean continent—an Atlantis—has sometimes been assumed 
in order to make it a source of the mud, sand and gravel, out 
of which the thick sedimentary formations of the Appalachian 
region of North America were made. But if this Atlantis 
were a reality, there would still have been needed, in addition 





GHOLOGICAL CONCLUSIONS. 389 


to the presence of such an ocean continent, a set of freight 
carriers that could beat off the waves from their accustomed 
work, and push aside the ordinary oceanic currents; or else 
; a would get back all its own dirt. 


VI. ABSENCE OF FOSSILS FROM LIMESTONE STRATA. 


Absence of fossils has been mentioned as a frequent char- 
acteristic of the fine compact coral reef-rock, and also of the 
beach and drift sand-rock or odlite (pp. 153, 194). The rocks 
are formed at the sea-level, and in the midst of abundant life, 
and yet trituration by the action of the waves and winds has 
in many places reduced all to the finest. material, so that an em- 
bedded shell is seldom to be found in the beach or drift odlite, 
and rarely too in inuch of the fine-grained coral reef-rock. 

The interior basins appear to be eminently the place for 
making these non-fossiliferous limestones. This is the case in 
two widely different conditions: fist, over the portions that are 
below the coral-growing depths, which are sometimes of great 
area; and second, im lagoons that have become so small and 
shallow that corals and large shells have all disappeared, and 
the trituration is of the finest kind, producing calcareous mud ; 
such lagoons being properly in a marsh condition. These last 
appear to illustrate on a small scale the conditions under which 
many of the ancient non-fossiliferous, or sparingly fossiliferous, 
limestones were formed. 


VII. THE WIDE RANGE OF THE OLDER LIMESTONES NOT EXEMPLIFIED 
AMONG MODERN CORAL-REEF FORMATIONS. 

Coral-reefs, though they may stretch along a coast for scores 

of miles, are seldom a single mile in width at the surface ; and if 

elevated above the sea, they would stand as broad ramparts 

separated by passages mostly 20 to 200 feet deep, and often of 


590 CORALS AND CORAL ISLANDS. 


great width. The substratum, however, is, in general, contin- 
uous coral-rock; and if these more elevated parts were re- 
moved by any process, after an elevation, they would leave a 
nearly level area of coral limestone often as extensive as the 
whole reef-grounds. In an island like Dean’s, one of the Pau- 
motus, these reef-grounds are 1,000 square miles in extent. 
Still greater are the Bahama banks, the largest of them being 
390 miles long and 200 wide, an intervening “tongue of the 
ocean’ excepted. 

But the most extensive reef-grounds of the oceans are after 
all of small breadth compared with many of the ancient lime 
stones of the continents; and the reef-rocks also are peculiar 


in their very abrupt limits, the margins sometimes descending 


at a steep angle a thousand feet or more. ‘These differences 
between the new and the old arise in part from the fact that 
the coral reefs of the present era are made about small oceanic 
lands, or along the edges of the continents, while the limestones 
of ancient time were generally formed over the broad surface 
of a continent as it lay slightly submerged. 4 

The Abrolhos reefs of the Brazilian coast, described on page 
140, illustrate one of the methods by which the coral banks ex- 
tend and finally coalesce into beds of wide extent; but these 
are small compared with the great limestones of early time, 
and owe their slight approximation to them as regards extent 
to the wide range of shallow waters there afforded. These 
Abrolhos reefs differ from most limestone beds also in being 
formed largely of the corals in the position of growth. 

The tendency of modern reefs to grow up to the surface in 
narrow banks, separated by channels, appears to be unlike any 
thing we discover in the old rocks; and it seems to be an un- 
avoidable result of growth in the sea, where the waves pile up 
barriers, and the currents make, and keep open, channels. The 

case of the Australian and Feejee reefs are good examples It 





GEOLOGICAL CONCLUSIONS. 391 


is possible that such barriers may often have existed in ancient 
time, and have disappeared through subsequent denudation of 
the surface. But may not the difference between the great even 
layers of the continental formations and those of a coral island 
have proceeded from the difference in the depth of the seas? 
Over the great shallow continental seas where the limestones 
were in progress, the waves may have generally been feeble, 
and therefore there may have been a less tendency to form nar- 
row barriers and deep intervening channels. 

The marsh condition of a drying-up lagoon with its forming 
limestones has been compared above with that under which an- 
cient unfossiliferous limestones were made. The narrow limits 


of the former make the comparison unsatisfactory ; for, in the 


coral island, coarsely fossiliferous beds are all the while form- 
ing about the exterior of the island, but a few miles at the 


most from the lagoon-marsh ; while the ancient limestones re- 


tain their unfossiliferous character often through many thou- 
sands of square miles. Still, the above mentioned difference be- 
tween the continental sea and the existing deep oceans may 
perhaps account for the diversity of results. 


VIII. CONSOLIDATION OF CORAL ROCKS. 


All true coral-reef rocks are examples of the consolidation 
of material mainly of coral origin—either mud, sands, frag- 
ments, or standing corals, the last with mud or sands inter- 
mixed—by (1), an under-water process; (2), at the ordinary 
temperature ; and they exemplify the mode in which all other 
submarine limestones of organic origin have been consolidated. 
The process appears to depend on the presence (proved by 
chemical analysis) of carbonic acid in the sea waters that bathe 
and penetrate the sands. This carbonic acid is derived from 
three sources: from (1), the rains which wash it down from 


392 CORALS AND CORAL ISLANDS. 


the atmosphere; (2) the respiration of all the animal life in 
the waters, even down to the simplest and minutest; and (3) 
the decomposition of all vegetable or animal débris in the 
waters or diffused through the sand or muds. This gas is set 
free, therefore, just where it is needed for the work, and is 
always ready to perform its part in the process of consolida- 
tion. It enables the water to take up carbonate of lime from 
the grains of the mass to be solidified, or from outside sources ; 
and then the deposition of the same among the grains through 
their attractions produces the cementation. 

The beach and drift sand-rocks or odlites are different 
from the reef-rock in being superficial deposits. The carbonic 
acid of the waters performs the same part as in the latter; but 
with these, there is alternate wetting and drying during the 


ebb and flow of the tides and the succession of gales and quiet ° 


winds. By this means, the grains become incrusted, and every 
new wetting and drying adds a new layer to the surface of each; 
and thus the odlitic structure is produced. [acts are men- 
tioned on page 153 of pebbles of volcanic or basaltic rocks, 
lying loose on a seashore, becoming incrusted in this way with 
a milky layer; and of basaltic conglomerates being made by 
the same means, the carbonate of lime being added until all 
the intervals between the stones were filled up and the whole 
made solid; and of an amygdaloidal volcanic rock on a coast 
having derived its little calcareous kernels or amygdules from 
the same source. ‘The following additional facts are cited 
from Mr. Darwin’s Journal (p. 588): 

“ Lieutenant Evans informs me that during the six years 
he has resided on this island (Ascension) he has always ob- 
served that in the months of October and November, when 
the sand [of a calcareous beach] commences travelling to- 
ward the southwest, the rocks which are situated at the end 


GEOLOGICAL CONCLUSIONS. 393 


of the long beach become coated by a white, thick, and very 
hard calcareous layer. I saw portions of this remarkable 
deposit, which had been protected by an accumulation of 
sand, In the year 1831 it was much thicker than during any 
other period. It would appear that the water charged with 
calcareous matter, by the disturbance of a vast mass of calca- 
reous particles only partially cemented together, deposits this 
substance on the first rocks against which it impinges. But 
the most singular circumstance is that, in the course of a 
couple of months, this layer is either abraded or redissolved, 
so that after that period, it entirely disappears. It is curious 
thus to trace the origin of a periodical incrustation, on certain 
isolated rocks, to the motion of the earth with relation to the 
sun; for this determines the atmospheric currents which give 
direction to the swell of the ocean, and this again the arrange- 
ment of the sea-beach, and this again the quantity of calcareous 
matter held in solution by the waters of the neighboring sea.” 

Mr. Darwin, speaking of a large beach of calcareous sand 
composed of comminuted and rounded fragments of shells and 
corals at Ascension, says, ‘‘The lower part of this, from the 
percolation of water containing calcareous matter in solution, 
soon becomes consolidated and is used as a building stone ; 
but some of the layers are too hard for fracture, and when 
struck by the hammer, ring like flint.” 

The surface of hills of drift sand-rock often has small de- 
pressions that are coated with a smooth, solid crust, as al- 


ready explained. 


IX. FORMATION OF DOLOMITE OR MAGNESIAN CARBONATE OF LIME. 

Analyses of the coral limestone of the elevated coral island 
Metia, by Prof. B. Silliman, Jr., have determined the singular 
fact that, although the corals themselves contain very little 


204 CORALS AND CORAL ISLANDS. 


carbonate of magnesia, magnesia is largely present in some 


specimens of the rock. The rock is hard (H. = 4), and splint. — 


ery in fracture, with a specific gravity 2°690. It affords 
on analysis, 38°07 per cent. carbonate of magnesia, and hence, 
only 61°93 of carbonate of lime. 

Another specimen from the same island, having the spe- 
cific gravity 2°646, afforded 5:29 per cent. of carbonate of mag- 
nesia. 

The former was a compact homogeneous specimen, and the 
latter was partly fragmentary. Recent examinations of coral 
sand and coral mud from the islands, give no different com- 
position, as regards the magnesia, from that for corals, which, 
as the analyses on page 99 show, contain very little or no 
magnesia. The coral sand from the Straits of Balabac, af- 
forded Prof. Silliman carbonate.of lime 98°26, carbonate of 
magnesia 1°38, alumina 0°24, phosphoric acid and silica a trace. 

This introduction of magnesia into the consolidating 
under-water coral sand or mud, has apparently taken place (1) 
in sea waters at the ordinary temperature; and (2) without 
the agency of any mineral waters except the ocean. But the 
sand or mud may have been that of a contracting and evap- 
orating lagoon, in which the magnesian and other salts of the 


ocean were in a concentrated state. It has been already ob- 


served (p. 349), that this was probably the actual condition 
of the elevated portion of the island of Metia, every thing 
about it looking as if it corresponded to the lagoon part of 
the old atoll; and also that the idea of the existence of min- 
eral springs there has no support in known facts. 


X. FORMATION OF CHALK. 


The formation of chalk from coral is known to be ex- 
emplified at only one spot among the reefs of the Pacific 








GEOLOGICAL CONCLUSIONS. ~\N 4395 


The coral mud often looks as if it might be a fit material for 
its production; moreover, when simply dried, it has much the 
appearance of chalk, a fact pointed out by Lieutenant Nel- 
son in his Memoir on the Bermudas (1834), and also by Mr. 
Darwin, and suggested to the author by the mud in the lagoon 
of Honden Island. Still this does not explain the origin of 
chalk ; for, under all ordinary circumstances, this mud _solidi- 
fies into compact limestone instead of chalk, a result which 
would naturally be expected. What condition then is neces- 
sary to vary the result, and set aside the ordinary process. 

The one locality of chalk among the reefs of the Pacific, 
referred to above, was not found on any of the coral islands, 
but in the elevated reef of Oahu, near Honolulu, of which reef 
it forms a constituent part. It is twenty or thirty feet 
in extent, and eight or ten feet deep. The rock could not be 
distinguished from much of the chalk of England : it is equal- 
ly fine and even in its texture, as earthy in its fracture, and so 
soft as to be used on the blackboard in the native schools, 
Some imbedded shells look precisely like chalk fossils. It con- 
tained, according to Professor Silliman, 92°80 per cent. of 
carbonate of lime, 2°38 of carbonate of magnesia, besides 
some alumina, oxyd of iron, silica, ete. 

The locality is situated on the shores, just above high-tide 
level, near the foot of Diamond Hill. This hill is an extinct 
tufa cone, nearly seven hundred feet in height, rising from the 
water’s edge, and in its origin it must have been partly sub- 
marine. It is one of the lateral cones of eastern Oahu, and 
was thrown up at the time of an eruption through a fissure, 
the lavas of which appear at the base. ‘There was some coral 
on the shores when the eruption took place, as is evident from 
imbedded fragments in the tufa; but the reef containing the 
chalk appeared to have been subsequent in formation, and 


296 © CORALS AND CORAL ISLANDS. 


afforded no certain proof of any connection between the fires 
of the mountain and the formation of the chalk. 

The fine earthy texture of the material is evidence that the 
deposit was not a subaerial seashore accumulation, since only 
sandstones and conglomerates, with rare instances of more 
compact rocks, are thus formed. Sand-rock making is the 
peculiar prerogative, the world over, of shores exposed to 
waves, or strong currents, either of marine or fresh water. We 
should infer, therefore, that the accumulation was produced 
either in a confined area, into which the fine material from a 
beach may have been washed, or on the shore of a shallow, 
quiet sea; in other words, under the same conditions nearly 
as are required to produce the calcareous mud of the coral 
island. But, although the agency of fire in the result cannot 
be proved, it is by no means improbable, from the position 
of the bed of chalk, that there may have been a hot spring at 
the spot occupied by it. That there were some peculiar cir- 
cumstances distinguishing this from other parts of the reefs, is 
evident. 

This, if a true conclusion, is to be taken, however, only as 
one method by which chalk may be made. For there is no 
reason to suppose that the chalk of the Chalk formation has 


been subjected to heat. On the contrary, it is now well ascer- — 


tained that it is of cold-water origin, even to its flints, and that 
it is made up largely of minute foraminifers, the shells of 
Rhizopods. Professor Bailey found under his microscope 
no traces of foraminifers, or of any thing distinctly organic, 
in the Oahu chalk. 


XI. RATE OF INCREASE OF LIMESTONE FORMATIONS. 


On page 253 it is shown that coral-reef limestones are of 
slow formation, the rate of increase in thickness, where all 


GEOLOGICAL CONCLUSIONS. 397 


is most favorable, not exceeding perhaps a sixteenth of an 
inch a year, or five feet in a thousand years. And yet such 
limestones probably form at a more rapid rate than those 
made of shells, because the animals are to a larger extent 
calcareous or make proportionally larger calcareous secretions ; 
and in addition they have the property of rapid multiplica- 
tion by budding. The mollusks that grow and multiply most 
rapidly and have proportionally the largest shells are the 
Lamellibranchs or bivalves, among which the oyster is a fa- 
mous example; and the Brachiopods were once the full equals 
of the ordinary bivalves. Large banks of bivalves seldom 
occur in regions of corals, the species there being to a great 
extent Gasteropods (or univalves) ; and hence the contributions 
of shells to coral reefs from mollusks are small compared 
with the extent of the beds which, by themselves, they make 
on other coasts. The coral seas of Florida nowhere have 
shore shell-beds like those of St. Augustine in northern 
Florida outside of the coral-reef seas. There is reason for 
this in the fact that these bivalves that grow in large banks live 
in beds of ordinary sand or mud, such as reef-regions do not 


generally supply. 


XII. LIMESTONE CAVERNS. 


The elevated coral limestone, although in general a hard 
and compact rock, abounds in caverns. They may be due in 
part to open spaces, or regions of loose texture, in or between 
the strata. But in most cases they are a result of solution 
and erosion by the fresh waters of the land, or the waves and 
currents of the ocean, subsequent to the elevation. 

On the island of Metia, many caverns open outward in 
the coral limestone cliff and in some were large stalactites, as 
stated on page 194. 


398 CORALS AND CORAL ISLANDS. 


In the raised coral rock of Oahu (p. 871) there are several 
long winding horizontal chambers, some of which are the 
sources of subterranean streams that open out on the shores 
between the layers of the rock, or from the mouths of caverns. 
These running waters, and others trickling from above, are 
obviously the eroding agents that have made the caves. 

As briefly remarked on page 194, caverns are still more re- 
markable on the island of Atiu, on which the coral reef-rock 
stands at about the same height above the sea as on Oahu, 
Rev. John Williams states that there are seven or eight of 
large extent on the island. Into one he entered by a descent 
of twenty feet, and wandered a mile in one only of its branches 
without finding an end ‘‘ to its interminable windings.” He says, 
“TInnumerable openings presented themselves on all sides as 
we passed along, many of which appeared to be equal in height, 
beauty and extent to the one we were following. The roof, a 
stratuin of coral rock fifteen feet thick, was supported by massy 
and superb stalactitic columns, besides being thickly hung with 
stalactites from an inch to many feet in length; some of these 
pendents were just ready to unite themselves to the floor, or to 
a stalagmitic column rising from it. Many chambers were 
passed through whose fretwork ceilings and columns of stalac- 
tites sparkled brilliantly, amid the darkness, with the reflected 
light of our torches. The effect was produced not so much by 
single objects, or groups of them, as by the amplitude, the 
depth, and the complications of this subterranean world.” 

Other similar caves exist on the neighboring island of 
Mauke. 

The Bermudas and Bahamas are also noted for their cay- 
erns. The great height of the easily eroded drift sand-rock 
gives these reef-regions a chance for caverns, large and small ; 
a notice of the Bermuda caverns will be found on page 224. 


... 


GEOLOGICAL CONCLUSIONS. 399 


These are examples of the comparatively rapid formation 
of caverns. The waters which run or percolate through them 
must be charged with carbonic acid to accomplish such work, 
and yet they have no source for this ingredient except the atmos- 
phere, animal respiration, and vegetable and animal decom- 
position in the soil. The flutings and stalactitic incrustations 
of a precipice facing the sea must depend on the former alone, 
with the aid perhaps of the spray from the sea brought over 
the reef by storms. 


XIII. OCEANIC TEMPERATURE. 


Facts seem to indicate—though perhaps not sufficient to 
demonstrate—that the Gulf Stream has had, from the Juras- 
sic period in Geological history onward, the same kind of in- 
fluence on the temperature of the North Atlantic Ocean which 
it now has. 

The existence of a coral reef made out of corals of the As- 
trea tribe and others, during part of the Odlitic era (middle 
Jurassic), in England, as far north as the parallel of 52° to 
55° is strong evidence that the isocryme of 68° F., the coral- 
reef boundary, extended then even to that high latitude; for 
species of the Astrea tribe are now confined to coral-reef seas 
(p. 109). This isocryme now reaches along the course of the 
Gulf Stream, to a point just north of the Bermudas, near 
33° N.; and 55° is 22° beyond this. 

There are no marine fossils in any rocks of that period on 
the American side of the Atlantic, so that facts fail for defi- 
nitely locating the western terminus of this odlitic isocryme of 
68° F. But it is highly improbable that the whole ocean 
across, on, or near, the parallel of 55° N., should have had, as the 
mean temperature for the coldest month of the year, one so high 
as 68° F’.; the present average position of the isocryme of 68°F... 


400 CORALS AND CORAL ISLANDS. 


through the middle of the two oceans, the Pacific and Atlan- 
tic, is near the parallel of 27° or 28°, or one-half nearer the 
equator than the parallel of 55°. It is difficult to account for 
an oceanic temperature high enough to give England’s seas 
68° FI’. as the average for the coldest winter month, even sup- 
posing the Gulf Stream to have aided; but it is vastly more 
difficult if no such northeastward current existed, and the high 
temperature extended equably so far from the equator. The 
probability is therefore strong that the existence of coral reefs 
in the Odlitic era in England was owing to the extension, by 
the aid of the Gulf stream, of the isocryme of 68° move than 
30° in latitude (and over 3,000 miles in distance) beyond its 
present most extra-tropical position, just outside of the Ber- 
mudas; in other words, that the whole ocean was just enough 
warmer, to allow this oceanic current (part of the great 
water-circulation of the globe) to bear the heat required for 
corals as far north as northern England. 

The present isocryme of 44° F’., as drawn on the chart of 
the world accompanying this volume, has approximately the 
course which that of 68° F. probably had in Odlitic times. It 
should have a little less northing, and the loop to the north 
should lean more to the eastward. The latter would have been 
a consequence of the submerged condition at the time of most 
of the European continent. 

The ocean’s waters seem to have cooled somewhat before 
the next period—the Cretaceous—began, since evidence fails 
of any Cretaceous coral reefs in the British seas; but such 
reefs prevailed then in central and southern Europe, so that 
the amount of cooling in the interval since the Odlitic era, 
had not been large; and as late as the Miocene Tertiary, there 
were reef corals in the seas of Northern Italy, above latitude 
45° N., or that of Montreal, in Canada. 





THE OCEANIC CORAL-ISLAND SUBSIDENCE. 40] 


The absence from the American coast of the Atlantic, 
of any coral reefs in the Cretaceous beds, and of any reef corals, 
seem to show that the oceanic temperature off this coast was 
not favorable for such corals; and if so, then the line of 68° | 
F’. extended at least 20° farther north on the European side! 
of the ocean, than on the Atlantic—an inequality to be ac- 
counted for in part by the existence of the Gulf Stream. But, 
in addition, the whole range of life in the European Creta- 
ceous, and its vastly greater variety of species, leave no doubt} 
as to the higher temperature of the ocean along its European 
border; so that the idea of a Cretaceous Gulf Stream must be 
accepted. And that of a Tertiary is demonstrated by similar 
facts. 

If the Gulf Stream had its present position and force in 
Odlitic, Cretaceous and Tertiary times, then the ocean had, 
throughout these eras, its present extension and oceanic char- 
acter; and, further, no barrier of land extended across from 
South America to the Canaries and Africa, dividing the South 
from the North Atlantic, but all was one great ocean. | 
Such a barrier would not annul entirely the flow of the Gulf 
Stream; yet the North Atlantic is so small an ocean, that if 
left to itself, its system of currents would be very feeble. 


XIV. THE OCEANIC CORAL ISLAND SUBSIDENCE. 


Coral islands have been shown to be literally monuments 
erected over departed lands; and, through the evidence from 
such records, it is discovered that the Pacific has its deep-water 
mountain chains, or lines of volcanic summits, not merely 
hundreds, but thousands of miles in leneth. Some of the 
ranges of high islands are proved by such records to have an 
under-water prolongation, longer than that above water: the 
Hawaian Islands for example, which have a length of only 


4()2 CORALS AND CORAL ISLANDS. 


four hundred miles from Hawaii to Kauai, and five hundred 
and thirty to Bird Island, the western rocky islet of the group, 
stretch on westward, as the coral registers show, even to a dis- 
tance of two thousand miles from Hawaii, or, as far as from 
New York to Salt Lake City; and how much farther is un- 
known, as the line of coral islands here passes the boundary 
of the coral reef seas, or the region where coral records are 
possible. 

Other ranges of submerged summits are shown to extend 
through the whole central Pacific, even where not a rocky 
peak remains above the surface; for all the coral islands from 
the eastern Paumotus to Wakes’ Island, near long. 170° E,. 
and lat. 19° N., north of the Ralick and Radack (or Marshall) 
groups, are in linear ranges; and they have, along with the 
equally linear ranges of high islands just south, a nearly uni- 
form trend, curving into northwest and north-northwest at the 
western extremity. The coral islands consequently cap the sum- 
mits of linear ranges of elevations, and all these linear ranges 
together constitute a grand chain of heights, the whole over five 
thousand miles in length. Thus, the coral islands are records 
of the earth’s submarine orography, as well as of slow changes 
of level in the ocean’s bottom. 

This coral island subsidence is an example of one of the 
great secular movements of the earth’s crust. ‘The axis of the 
subsiding area—the position of which is stated on page 363, 
has a length of more than six thousand miles—equal to one- 
quarter of the circumference of the globe; and the breadth, 
reckoning only from the Sandwich Islands to the Friendly 
Group (or to Tongatabu) is over twenty-five hundred miles, 
thus equalling the width of the North American continent. 
A movement of such extent, involving so large a part of the 
earth’s crust, could not have been a local change of level, but 


Se 


THE OCHANIC CORAL-ISLAND SUBSIDENCE. 4(1)3 


one in which the whole sphere was concerned as a unit; for 
all parts, whether participating or not, must have in some 
way been in sympathy with it. 

This subsidence was in progress, in all probability, during 
the Glacial era, the thickness of the reefs proving that in their 
origin they run back through a very long age, if not also 
into the Tertiary. It was adownward movement for the Trop- 
ical Pacific, and perhaps for the warmer latitudes of all the 
oceanic areas, while the more northern continental lands, or at 
least those of North America, were making their upward 
movement, preparatory to, or during that era of ice. 

The subsidence connected with the origin of coral islands 
and barrier reefs in the Pacific has been shown (p. 357) to have 
amounted to several thousands of feet, perhaps full ten thou- 
sand. And, it may be here repeated that, although this 
sounds large, the change of level is not greater than the eleva- 
tion which the Rocky Mountains, Andes, Alps and Himalayas 
have each experienced since the close of the Cretaceous era, 
or the early Tertiary; and perhaps it does not exceed the 
upward bulging in the Glacial era of part of northern North 
America. 

The author has presented reasons for beleving (Am. J. Sci., 
III. v. 1873) that in this Glacial era the watershed of Canada, 
between the River St. Lawrence and Hudson’s Bay, was 
raised somewhat above its present level (1,500 feet); and 
that this plateau thus elevated was the origin of the great 
glacier which moved southeastward over New England. This 
region is the summit of the eastern arm of the great V-shaped 
Archean area of the continent, the earliest elevated land of 
North America ; and it is not improbable that the other arm of 
the V, reaching from Lake Superior and Huron, northwest- 


ward, to the Arctic, was the source of glacial movements over 


404 CORALS AND CORAL ISLANDS. 


the more central portions of the continent :—we cannot say 
western portions also, since in the first place, the facts, accord- 
ing to Prof. J. D. Whitney, do not sustain the statement ; and, 
in the second, the great mountain ranges of the west would 
have been a barrier to all influences from any central conti- 
nental elevation, and, besides, the slopes of these ranges, even if 
the Pacific border were higher to the north than now, would 
have determined the course of all western glacial movements. 

The idea that both arms of the great Archzean nucleus were 
raised together, is not without some support. For the courses of 
the two were the courses of great continental uplifts or move- 
ments, again and again, through the successive subsequent ages; 
and the present outline of the continent is but the final expres- 
sion of the great fact; moreover, the elevations parallel to the 
western arm of the V have been much the greatest. Even the — 
exceptional courses, such as the nearly north and south trend of 
the Green Mountains, were marked out first in the Archzean, the 
Archzean peninsula of northern New York with the line of the 
Adirondacks being an exhibition of it. And all this uniformity 
of movement, from the laying of the first stone in the develop- 
ing continent to the last, has been shown by the author to be 
directly connected with the fact that the continent has always 
been bordered by the same two great oceanic depressions, the 
Atlantic, and the larger Pacific, the same in trend of axis as 
now, the North Atlantic having a northeast and southwest 
trend, parallel with one arm of the Archean, and the Pacific 
a northwest and southeast parallel with the other arm of the 
Archean. It is therefore reasonable that, late in geological 
history, during the Glacial era, after the great mountain chains 
of the continent had been made and raised to their full height, 
and the surface crust thickened over all the continent, except 
that of the Archzan nucleus, by successive beds to a thickness of 


THE OCEANIC CORALISLAND SUBSIDENCE. A405 


thousands of feet, even thirty-five thousand by the close of the 
Paleozoic along the Appalachians, and much beyond this on 
the Pacific border; and when these thick sediments had in 
many regions been stiffened by crystallization or metamor- 
phism; I say it is reasonable that, finally, changes of level, 
through the working still of the old system of forces, should 
again have affected most the old nucleal Archzan area of the 
continent, where there had been no thickening except what 
had taken place internally; and that, if one arm of the V, 
that along the Canadian watershed, were raised at this time, 
the other, northwestern in trend, should also have been raised. 
This is at least probable enough to become a question for 
special examination over the region. See further the author's 
Manual of Geology, 1874. 

The northern continental upward movements which intro- 
duced the Glacial era, carrying Arctic cold toward the tropics, 
may have been a balance to the downward oceanic move- 
ments that resulted in the formation of the Pacific atolls. While 
the crust was arching upward over the former (not rising into 
mountains, but simply arching upward) it may have been 
bending downward over the vast central area of the great ocean. 

The changes which took place, cotemporaneously, in 
the Atlantic tropics, are very imperfectly recorded. The 
Bahamas show by their form and position that they cover 
a submerged land of large area stretching over six hundred 
miles from northwest to southeast. The long line of reefs 
and the Florida Keys, trending far away from the land of 
southern Florida, are evidences that this Florida region par- 
ticipated somewhat in the downward movement, but to a 
much less extent than the Bahamas. Again, the islands 
of the West Indies diminish in size to the eastward being 
quite small in the long line that looks out upon the blank 


’ 406 ve CORALS AND CORAL ISLANDS. 


ocean, just as if the subsidence increased in that direction. 
Finally, the Atlantic beyond is water only, as if it had been 
made a blank by the sinking of its lands. 

Thus the size of the islands, as well as the existence of 
coral banks, and also the blankness of the ocean’s surface, all 
appear to bear evidence to a great subsidence. 

The peninsula of Florida, Cuba, and the Bahamas look, as 
they lie together, as if all were once part of a greater Florida 
or southeastern prolongation of the continent. The northwest- 
ern and southwestern trends, characterizing the great features 
of the American continent, run through the whole like a warp 
and woof structure, binding them together in one system; the 
former trend, the northwest, existing in Florida and the Ba- 
hamas, and the main line of Cuba; and the latter course, the 
west-southwest, in cross lines of islands in the Bahamas (one 
at the north extremity, another in the line of Nassau, and 
others to the southeast), in the high lands of northwestern 
and southeastern Cuba, and in the Florida line of reefs, and 
even further, in asubmerged ridge between Florida and Cuba. 
This combination of the two continental trends shows 
that the lands are one in system, if they were never one in 
continuous dry land. 

We cannot here infer that there was a regular increase of 
subsidence from Florida eastward, or that Florida and Cuba 
participated in it equally with the intermediate or ad- 
joining seas; for the facts in the Pacific have shown that 
the subsiding oceanic area, had its nearly parallel bands of 
greater and less subsidence, that areas of greatest sinking al- 
ternated with others of less, as explained on page 326; and that 
the groups of high islands are along the bands of least sinking. 
So in the Atlantic, the subsidence was probably much greater 
between Florida and Cuba than in the peninsula of Florida it 


THE OCEANIC CORAL ISLAND SUBSIDENCE. 407 


self; and greater along the Caribbean Sea parallel with Cuba, 
as well as along the Bahama reefs, than in Cuba. 

The conclusions of Mr. Thomas Bland, based on the distri- 
bution of terrestrial mollusks in the Bahama Islands, a sub- 
ject with which he had made himself familiar by study in 
the region, have much interest in this connection. He 
shows? that these mollusks, eighty in number of species, 
prove that the alliance of the Great Bahama Bank with 
Cuba is very close, as is apparent in the many species of 
Polymeta and Strophia, and the occurrence in both of the 
genera Polygyra, Thelidomus and Melaniella, not known in 
Hayti; while Turk Island bears evidence of connection with 
Hayti through the genus Plagioptycha and the species com- 
mon to the two, P. Albertsiana and P. disculus. 

After mentioning the views in this volume on the dimin- 
ished size of the islands to the eastward and the evidence 
thereby of subsidence, he says : — 

“The facts regarding the diminution in size of the islands 
of the West Indies to the eastward are of peculiar interest, not 
only as affording conclusive evidence of greater subsidence in 
that direction, but also in connection with geographical distri- 
bution. The banks and islands forming the long Bahama 
chain diminish in size to the southeast, where are situated at 
its termination the submerged Mouchoir Carré, Silver and 
Navidad Banks. In a similar manner, the submerged Virgin 
Island Bank (with Anegada on its northeastern extremity 
geologically resembling the Bahamas in the opinion of Dr. 
Cleve), Sombrero and the Anguilla Bank, terminate the chain 
of the West Indies eastward from Cuba, parallel with the Ba- 
hama chain. In the caves of Anguilla the remains of large 


1 Annals of the Lyceum of Natural History of New York, Vol. X., 1873, an 
abstract of which appeared in the American Journal of Science, 1874, VIII. 231. 


408 CORALS AND CORAL ISLANDS. 


extinct Mammalia are found which must have inhabited a far 
more extensive area subsequently broken up by subsidence.” 

The position of the lonely Bermuda atoll confirms these 
deductions. Its solitary state is reason for suspecting that 
great changes have taken place about it; for it is not natu- 
ral for islands to be alone. The tongue of warm water due 
to the Gulf Stream, in which the Bermudas lie, is narrow, 
and an island a hundred miles or more distant to the north- 
east-by-east, or in the line of its trend (p. 219), if experiencing 
the same subsidence that made the Bermuda land an atoll, 
would have disappeared without a coral monument to bear 
record to its former existence. Twenty miles to the south- 
west-by-west from the Bermudas, there are two submerged 
banks, ten and twenty-four fathoms under water, showing 
that the Bermudas are not completely alone, and demonstrat- 
ing that they cover a summit in a range of heights; and it 
may have been a long range. 

In the Indian ocean, again, there is evidence that the 
coral-island subsidence was one that affected the oceanic area 
more than the adjoining borders of the continent, and most, 
the central parts of the ocean. For, in the first place, the 
Archipelago of the Maldives narrows and deepens to the 
southward (p. 186). Further, the large Chagos Group, lying 
to the south of the Maldives, contains but very little dry 
land in any of its extensive reefs, while some of them, includ- 
ing the Great Chagos Bank, are sunken atolls. Again, still 
other large reefs nearly bare lie to the southwest of the 
Chagos Bank, and submerged banks exist in the seas north- 
east and east of northern Madagascar. 

The probability is, therefore, that both the central Atlan- 
tic and Indian Oceans included regions of subsidence like the 
central Pacific, and that the absence of islands over a large 


THE OCHANIC CORAL-ISLAND SUBSIDENCE. 409 


part of their interiors may be a consequence of it. A rate of 
sinking exceeding five feet in a thousand years (if the estimate 
on page 253 is right) would have buried islands and reefs to- 
gether in the ocean; while, with a slower rate, the reefs might 
have kept themselves at the water’s surface. So small may 
have been the difference of rate in the great movement that 
covered the Pacific with coral islands, but left the Indian 
Ocean a region of comparatively barren waters, with some 
“half-drowned” atolls, and the central Atlantic almost wholly 
a blank. 

While thus seeming to prove that all the great oceans have 
their buried lands, we are far from establishing that these 
lands were oceanic continents. For as the author has elsewhere 
shown, the profoundest facts in the earth’s history prove that 
the oceans have always been oceans. These lands in all proba- 
bility were, for the most part, volcanic islands or summits of 
volcanic ranges, for of this nature are all the islands over the 
interior of either ocean that are not of coral origin. 

The course of argument leads us to the belief that a very 
large number of islands, more than has been supposed, lie 
buried in the ocean. Coral islands give us the location of 
many of these lands; but still we know little of the extent to 
which the earth’s ranges of heights, or at least of volcanic 
peaks, have disappeared through oceanic subsidence. Recent 
dredgings and soundings have proved that the bottom of the 
oceanic basin has little of the diversity of mountain chains 
and vallies that prevails over the continents; and, through 
this observation (and also by the discovery that some ancient 
types of animal life, supposed to have been long extinct, are 
perpetuated there), they have afforded new demonstration of 
the proposition, above stated, that the oceans have always been 
oceans. But while the facts do not imply the existence deep 


q 


in the ocean of many granitic mountain chains, they do teach — 


410 CORALS AND CORAL ISLANDS. 


that there are long ranges, or lines, of volcanic ridges and 


peaks, and some of these may be among the discoveries of — 
future dredging expeditions. A range of deep-sea cones, or — 
sunken volcanic islands, would be as interesting a discovery — 


as a deep-sea sponge or coral, even if it should refuse, ex- 
cepting perhaps a mere fragment, to come to the surface in 
the dredge. 


We may also accept, with some confidence, the conclusion — 


that atolls and barrier reefs originated in the same great bal- 


ance-like movement of the earth’s crust that gave elevation — 
and cold, in the Glacial era, to high-latitude lands. If so, the — 


tropics and the colder latitudes were performing their several 
works simultaneously in preparation for the coming era; and 
it is a gain to us in our contemplations, that we hence may bal- 
ance the beauty and repose of the tropics, through all the pro- 
gressing changes, against the prolonged scenes of glacial deso- 
lation that prevailed over large portions of the continents. 





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APPENDIX. 





I. ARTESIAN WELLS ON SOUTHERN OAHU. 


On page 287 reference is made to artesian borings on Oahu of the Ha- 
waiian Islands, that descended through coral reef strata of much thickness 
and at large depths, which are believed to be good evidence of a former 
higher level of the island by some hundreds of feet, and therefore of a grad- 
ual subsidence since, the only doubt coming from the possibility that the coral 
rock at the lower levels may be not true reef rock but of pelagic origin. The 
following are the details as to fifteen of these borings in the area between 
the centre of the city of Honolulu and Diamond Head on the coast to 
the eastward. The level of the surface from which they start is not above 
thirty feet. 

The positions of these wells may be learned from the following map, — a 
reduced copy of a chart received from the surveyor general of the Hawaiian 
Islands, Prof. W. D. Alexander. Part of the shore region of Southern 
Oahu is here represented, from the city of Honolulu, on the west, to the 
tufa cone, Leahi or Diamond Head, whose southern brow has a height of 761 
feet. Puowaina, or Punchbowl, standing just back of Honolulu and 498 feet 
high, is also a tufa-crater. Rocky Hill, near Oahu College, and some low 
hills near by, appear to have been made by flows of lava from fissures of 
comparatively recent date. The large and deep valleys of the mountains 
Nuuanu and Manoa, and the narrower Palolo valley, open out on this shore- 
region near the northern limit of the map. 

The map also shows the position and width of the coral reef of the coast, 
and of the harbor of Honolulu, which owes its existence to the reef. The 
larger part of this shore-region is covered by the elevated fringing reef, which 
has a height above the sea of fifteen to twenty-five feet. Its surface is cov- 
ered by six to ten feet of a glassy black volcanic sand, which may have been 
ejected from the Rocky Hill vent, and four to six feet of surface soil. The 
borings descend through the soil and sand, and then the elevated fringing reef 
of coral limestone, to a bottom of solid lava, and intersect at some levels, 


besides coral rock, layers of tufa, lava, “clay,” sand or bowlders, which are 


412 APPENDIX. 


the results of occasional eruptions during the growth of the reef, or of depo- 
sition from the streams of the valleys. 

The positions of the artesian wells are indicated on the map by small 
circles, and those referred to beyond have their names attached in small 
capitals. They are also numbered within the circles. (A magnifying-glass 
may be needed to read the names on the much-reduced map.) The records 
of the superintendent of the borings, Mr. J. A. McCandless, were received 
through Prof. W. D. Alexander, excepting that of the Atherton well, for which 
and for specimens of the materials of the successive beds, the author is indebted 
to Mr. W. C. Merritt, President of Oahu College. 

The wells are topographically of three groups. Seven of them (numbers 
1 to 7) are situated around the base of Punchbowl; four (8 to 11) southwest 
of the base of Rocky Hill; and four (12 to 15) northwest to southwest of 
Diamond Head, the last one at its base, and only a short distance from the 
seashore. In order to interpret the sections it should be noted that the preva- 


lent winds under whose action the tufa cones were made are the trades, or | 


from the northeast. The thickness of beds and depth are given in feet. To 
aid in the comparison of the results, the sections that are most similar are 
placed in parallel columns. 


1. ARTESIAN WELLS ABOUT THE BASE OF PUNCHBOWL 

























GNos:/ 120: 7): 
1. Foster WELL, £. OF PUNCHBOWL. 2. PaLace WELL, s. or PUNCHBOWL. 
Thick- | 
et Depth. 
Soil 4 ft., black sand4 . 8 Soil 4, black sand 4 
Bowlders . . . A 6 14 Cora . A 72 
Lava ; cae 40 54 Lava. cae 78 
Clay 16, bowlders 12. . 28 82 WHITE CoRAL 138 
Clay 20, bowlders8 . . 28 110 
Claye ers *. se | 800 410 Clay . 378 
CORAL.) §. 40 450 CorAL 452 
Clay, gravel, ‘110; clay, 
bowlders, 180. . 290 | 740 Clay and gravel . 707 
Lava or bed-rock . . rat hoe Lava or bed-rock 762 








8. ATHERTON WELL, Ss. OF PUNCHBOWL. 











| | 
whi | pep 

Soil 6, black sand6 . . 12 Corat (clay 5 incl.) 
Cora (clay 5 incl. ) a oo || Lets) 190 Gravel and clay . 
uRediclayac) ue gaohs 10 200 CoraL Mei ae ts 565 
CorRaL . 30 230 Clay, sand, some coral, 
Brown clay (coral 6 5 incl. 60 290 gravel mAbs 595 
CORATAS ee 3 320 | Bed-rock or lava 655 
Browniclayaeem aces menes tS) 355 | 























HAWAIIAN GOVERNMENT SURVEY 


HONOLULU 


AND VICINITY 





ENTRANCE 
To 


S— 
Rece-tieiae [- 


Ne 
co 


eS 


{ GOO KIM 3 
Ox 





» 
PRR Litvak alent 


scurry 


xf 


Plate XVII. 


> S| 








Rta aka 





4 


414 


4. THomas SQuaRE WELL, S. OF PUNCH- 




















APPENDIX. 


6. Ice Co. (Sass’) WELL, s. oF Puncu- 

























































































BOWL. BOWL. 
Thick-_ Thick- 
mene | Depth. age Depth 
Soil 6, black sand 6, gas + 16 Soil 4, black sand 10 . 14 | 
CoraL . : 200 216 CoraL . . j 200 214 
Brown clay 44 | 260 IBroWwniClayee me. er ee 3 252 
Cora 10 | 270 Cora. SRG areas 25 277 
Brown clay 60 380 Clay 40 317 
WHITE CoRAL 50 | 380 CorRAL 80 397 
Brown clay 80 460 Clay . 53 450 
Bed-rock or lava 49 509 Lava, bed-rock | 53 503 
| 
5. Warp’s, s. OF PUNCHBOWL. 7. WrxLcox, s. of PUNCHBOWL. 
poe Depth. aS Depth. 
Soil and sand. 15 Soil 4, black sand 6 10 
Cora . 204 219 Coray . . 50 60 
Yellow clay ; 4 260 Hard lava (fr. Rocky Hill ") 40 100 
HARDICORAL. Gg. 51 cu 1 YAGI |) Clyro Ae 30 130 
Yellow clay 109 570 Corau 120 250 
CoraL . . 23 393 || Clay. 30 280 
White and yellow clay, CoRAL . 70 500 
sand . . 111 504 Clays 100 450 
Lava. 4 508 Bowlders, clay, gravel. 85 | 535 
Lava bowlders 40, clay 45 85 620 
Lava or bed-rock 40 660 
2. ARTESIAN WELLS SOUTHWEST OF ROCKY HILL. 
8. DiztiincHam WELL. 9. Darry WELL (SOUTHERN). 
ban Depth. apie Depth 
Loam, gravel, cay» 90 Soil, sand, sash 60 
Corat . : ‘ 40 130 CoRAL . ae iO) 70 
Clay . 60 190 Clay. . as 35 105 
CoraL . 20 210 Genus (clay 5 incl. ) 100 205 
Black clay . : 25 230 
Clay, black sand 40 250 Conran 2. 10 240 
Clay 30, sand, “gravel . 40 | 280 
Lava (fr. Rocky Hill *) 50 300 Lava, black (fr. Rocky 
Hil yee beara ae 10 | 290 
— — ——— — as — U - 
10. Darry WELL Ueaeues aN 11. Marques WELL. 
Thick Depth. br | Depth 
Soil? claw, 4). eeke oe 30 | Earth 10 | 
Bowlderclayies 0 = 90 120 | Sandy, soft Corat. 20 |. 730 
Coratandclay. . . 15 135 Lava, layers of gravel. 40. |" 30 
CoRAL . Bakes on 10 145 Clay. = ewes ahr) eee 3 100 
Clay, sand, ‘gravel che: 25 170 
Bed- rock, lata. ake. 3 213 Lava in layers 195 295 


























APPENDIX. 415 


The above four wells all bear evidence of their nearness to Rocky Hill, 
a lateral lava vent, and of their distance from the seashore. 


3. ARTESIAN WELLS NORTHWEST TO SOUTHWEST OF 
DIAMOND HEAD. 


12. Panos WELL, 6300 FEET FROM THE Coast. 




















Thick- | Depth. | pee | Depth. 
CIEST So ete RS aes an a7 Wet CORATy Rem hes! aN [3 50 175 
Bowlders 17, clay 10 . . 27 64 Clayaneta seine es t's 20 195 
Clay and coraL . : 8 72 CORA aerate eae 1. 80 275 
Bowlders: 4) - 00s: 2. 8 80 Clay, sand. . Viet: 100 375 
“COOTES pgek Re Geet Beate 26 106 Lava, or bed-rock . . . 32 407 
va . 19 125 











13. Goo Kim’s We tt, 4000 FEET FROM THE CoasT. 


3 %, l : 
Thick- | Thick- 
Wak | Depth. | ness. | Depth. 


























| as 

Soil 10, anyel 133 AAS aA 23 | Clays 203) en Gace te ee 20 290 
Lava. . ee 43 GON > Consu = aa ee Pee 50 344 
MEaPeE Un asi ne bor |) 124) Clays. co oo obs eh 20" S80 
Cora 5 Sh) bop als ee 26 150 WORAT@ htcak se Be fee 0 430 
eee et I 96> 1 ATEN “Clay Sis we ie vane eis 
GRATES a ee 94 270 || Lava, bed- rock bu fetes male 65 540 





14. Kine’s WELL, asoutr 1000 FEET FROM THE COAST. 








Thick- | 




















ness, | Depth. | aS Depth. 
Soilland coral ss). - 38 || Soft Cora, 30, white, 451 75 495 
RVhite. CORAL’ 9. -<. 4 22 GOmN SClay-euert aa. : 30 525 
ellow:sand) =) 0. * 43 103 Corarn, white; “27: 100 625 
Wie = Mo ene 47 150) |, Clay. (tough). 9: 2° =. 5 630 
White Conran... . 110 260 || Coranandclay. .. . 70 700 
Clay . . canto f wae: 120 380 Clay 28, black sand2. . 30 73 
Roe, hard'.0) 51 2s ee 40 420 || Lava,bed-rock . .. . | 120 850 





15. CAMPBELL’s WELL, NEAR THE Foot or D1AmMonp HEAD, ABouT A MILE MORE 
TO THE SoutTH THAN WELL No. 14. 





























ahi Depth. a ae Depth. 
Gravel, beach-sand. . 50 WHITE CORAL, soft . . | 28 1048 
Tufa, like that of Dia- Rock soft like soapstone. | 20 1068 
mond Head . . . 270 329 Brown clay, with broken | 
CoRAL, HARD, WHITE, | Corley has Renee ee eT O Sa eLnTS 
LIKE MARBLE. 9. .) | 605 825 || Lava. . 45 1223 
Dark brownclay .. . 75 900 Black clay 10, red pipe 
Washed gravel . . . . 25 925 | clay, 18> 28 =| 1251 
WP Prauaae ited, cumeeiact hme 249 1500 


Mery red clay)... .%: 95 | 1020 





1 5 foot layer of clay included. 


416 APPENDIX. 


The great distance of the Pahoa well from the coast is a reason for the 
small amount of coral rock. 


b] 


The water of Campbell’s well was “as salt as brine ;” it stood in the 


casing about a foot higher than the water of a surface well adjoining. 


The artesian wells indicate great diversity in the thickness of the coral 


formation, and a dependence, as regards thickness, on distance from the hills. 
Campbell’s well, No. 15, is about 4,000 yards from the hills, and has a lime- 
stone bed of “hard coral rock, like marble” over 500 feet thick, which at 
bottom is 865 feet below the sea-level, besides a bed of probably similar origin 
over 200 feet lower; while King’s well, about one half nearer the hills has the 
bottom of the lowest coral bed at 700 feet. ‘The same level in Goo Kim’s 
well, No. 13, is at 430 feet, and in the Pahoa well, No. 12, still nearer the 
hills (the distance but 1,500 feet), 275 feet. All these levels are below the 
limit of growing corals, and far below the Hawaiian limit, which, according 
to Mr. Agassiz, is less than 100 feet. But the intercalation of beds of lava, 
tufa, and clay (either tufa beds or decomposed lava) make a close comparison 
of the sections in this respect with one another impracticable. 

The wells about Punchbowl have great interest. The Foster and Palace 
wells, Nos. 1 and 2, are about 600 yards from the base of Punchbowl, and 
bear N. 70° W., and S. 65° W., from its centre. In each, the bottom bed of 
coral extends to a depth of about 450 feet (450 in No. 1, and 452 in No. 2), 
which, as No. 2 shows, is 442 feet below the top of the elevated fringing reef. 

Hence, in the case of each, the coral reef rock at bottom is more than 350 
feet below the Hawaiian limit of growing corals. Difference in position must 
account for the difference in the upper portion of the two sections; and the 
chief fact as to position is that the Foster well is within the Nuuanu Valley, 
where clay and bowlders may have been carried down by its running waters. 
But in an important feature they are alike, the coral bed just referred to hav- 
ing over it, in one, 300 feet of “clay,” and in the other, 278 feet. This thick 
bed of “clay ” was very probably made by cinder-ejections from Punchbowl; 
for the tufa of Punchbowl (a kind of palagonite), if brought up from a boring, 
would feel and look much like clay. This is further sustained by the fact that 
in the wells more to the south of Punchbowl, little of the clay-bed was found, 
the localities being too far to the eastward to receive much of the cinders; 
their bearings from the centre of Punchbowl are between S. 30° W., and 
Spiel De 

In each of the five wells, Nos. 3 to 7, the top layer consists of 12 to 16 
feet of soil and black sand. Below this, in Nos. 3 to 6, there are, severally, 
178, 200, 200, 204 feet of the coral limestone of the elevated fringing reef ; 
and the lower limits of the bottom layer of coral are, severally, at 515, 380, 
397, 393 feet. In the Wilcox well, No.7, a bed of lava, 40 feet thick (perhaps 


APPENDIX. 417 


from Rocky Hill) with 30 feet of “ clay” below it, intervenes between the 
first and second beds of coral limestone, which two have together a thickness 
of 170 feet. It is an interesting example of the adverse circumstances attend- 
ing coral growths about an island of active fires. 

The author is unable to find in the facts from these wells evidence that 
sustains, as urged by Mr. A. Agassiz,’ the Murray hypothesis, or anything 
that sets aside the various objections to this hypothesis that have been pre- 
sented. In two of the Oahu wells, the Jaeger well and one of the Govern- 
meut wells on the Waikiki road, according to Professor Alexander, carbonized 
cocoanut-wood was found at a depth of about 150 feet, beneath a 150-foot 
stratum of coral. ‘This and all the other observations in connection with the 
wells are fully explained by the theory of subsidence. 

Oahu is an example of an island that has had an upward shove, notwith- 
standing the progressing subsidence. The amount of elevation indicated by 
the elevated coral reef is about 25 feet on the south side and 50 to 60 feet on 
the north side. The Kahuku bluffs of the vicinity of the north cape are not 
made wholly of drift sand, as Mr. Agassiz concluded from his observations. 
The bluffs have a top layer of wind-drift origin, while the rest, 50 to 60 feet 
high above the sea-level by estimate, is true coral reef rock, as the author has 
illustrated elsewhere. Such a change of level, as already stated, is not 
against the subsidence theory ; it is one of the common incidents of a volcanic 
region.” 


Il. RATE OF GROWTH OF CORALS AND CORAL REEFS. 


Arrangements made at Tahiti for measuring the rate of growth of a coral 
reef. — The arrangements made by Captain Wilkes for measuring the rate 
of growth of coral reefs are mentioned on page 257. A memoir by MM. Le 
Clerc and De Benazé was published at Paris in 1872, giving an account of 
their attempts to make use of the stone planted by Captain Wilkes. They 
made various measurements; but they observe that Wilkes does not state 


1 Coral Reefs of the Hawaiian Islands, by Alexander Agassiz, Bulletin of the Mu- 
seum of Comparative Zoology, 1889, X VII., 121. 

2 The fact that corals were growing about Oahu during the time when tufa eruptions 
were in progress over the southern border of the island, is proved by facts recently com- 
municated (Dec. 11, 1889) to Professor Alexander, by Rey. 8. E. Bishop, of Honolulu. 
He observes that in Halawa, Ewa, at the cutting for the railway across two small bays of 
Pearl Lochs, and south of Kuahua Island, there occur, in the finely laminated tufa, layers 
of comminuted shells and corals. Mr. Bishop concludes that these materials were “ prob- 
ably torn off from the sides of the fissure of ejection that was presumably opened through 
the anciently subsided strata of corals and shells.” Fragments of corals and shells were 
still more abundant over the top of the tufa. Scattering pieces of coral were also observed 


by Mr. Bishop imbedded in the tufa of the low craters southwest of Koko Head. 
27 


418 APPENDIX. 


whether he measured from the top of a head of coral, or from the solid bank 
on which the corals were growing: and, further, that the use of our “ excel- 
lent spirit level” from a stone of so little length is not sufficiently exact for 
correct results, and hence they draw no conclusions from their trials. 

Before leaving the region, they made the following arrangements with 
reference to future measurements. They planted two blocks of coral, cement- 
ing them below, nearly burying them in the soil, placing them 0.21 metres 
above the Wilkes’ stone, which is between them. ‘They then put a mark 
upon them on plates of metal directed toward the place of observation on the 


shoal. A third stone was placed forty metres from the southwest angle of 


the Point Venus lighthouse, in order to give a second observation on the 
position of the spot on which the soundings were to be made. This spot was 
found to bear from the two new stones N. 77° 30’ E.; from the third stone, 
N. 70° 55’ E ; from the bell of the new Mission Church, S. 81° 40! E. A 
horizontal line passing from the mark on the new stone is 7.460 metres above 
the madreporic heads. 

They also made observations which satisfied them that Tahiti was not at 
present undergoing any general elevation. ‘Two maps accompany the pam- 
phlet: one is copied from Wilkes; the other is from a chart by Lieutenants 
Le Clere and Minier, and contains lines showing the positions of the points 
referred to above. See page 246. 

The following letter on the Rate of Growth of Florida Corals, for which 
the author is indebted to Mr. H. T. Woodman, the investigator of the subject, 
was received too late for the use of the facts in the earlier part of this work. 
It is dated :— 


BurrFato, N. Y. Jan. 25, 1890. 


** Herein I enclose a copy from the diary which I kept while on the Florida 
Reef during the winter of 1881-82. It gives almost the exact growth of such 
massive corals as I found or placed in a shallow channel across the reef just 
east of the Tortugas Group in the winter of 1867-68. The water on the reef 
at this point was only about three feet deep with two feet more in the chan- 
nel at lowest tide. From the healthy appearance of the Madrepora cervi- 
cornis on both sides of the channel, with Miilepora alcicornis and Porites 
clavaria in close proximity, as well as good specimens of Meandrina labyrin- 
thica, Orbicella cavernosa, and a Dichoccenia within a few feet of the centre 
of the channel, I was inclined to think that no more favorable locality to 
watch the growth of some of the massive corals could be found. I therefore 
added to the above-named massive forms, a Siderastrea, Orbicella annularis, 
Porites astreoides, Diploria cerebriformis, Manicina areolata, Meandrina 


sinuosa, and M. clivosa. 








APPENDIX. 419 


In the winter of 1881-82, the last time they were examined, their growth 
for the fourteen years was as follows : — 

Dichoceenia had an upward growth of a fraction less than half an inch; 
Orbicella cavernosa, seven eighths of an inch; O. annularis, one and a quarter 
inches large; Siderastrzea, a fraction less than five eighths of an inch; Diplo- 
ria cerebriformis, almost three fourths of an inch; Meandrina sinuosa, an 
inch and a quarter large; M. labyrinthica, an inch and seven eighths; Mani- 
cina areolata, a fraction less than five eighths of an inch. 

We could not get the exact rate of growth of the Mzandrina clivosa for 

‘the reason that it was not regular. The specimen, when placed in the chan- 
nel, measured less than six inches and in the fourteen years had spread out 
over eight inches, being, when last measured, about fourteen inches in diame- 
ter, while its upward growth was three or four inches in some places and less 
than an inch in others; and what appeared still more strange was the fact 
that the thickest piece was near the edge. I regard the growth of this species 
very uncertain. I have frequently seen it on bricks and other objects in the 
form of an incrusting coral and measuring six or more inches in diameter with 
perhaps less than half an inch in thickness. It is the most abundant coral at 
Fort Taylor as well as at Fort Jefferson. 

Madrepora cervicornis had so encroached upon the channel as to oblit- 
erate all of my marks, hence I know but little of its rate of growth; but it is 
certain that the channel had been narrowed from six to eight or perhaps ten 
feet by this coral. 

The temperature of the water has much to do with the growth of some 
species of corals. I do not now recall a single instance of finding a specimen 
of Dichoccenia or of Orbicella cavernosa, except in close proximity to the Gulf 
Stream. The largest specimen of O. cavernosa I have ever found was in a 
four foot channel where the waters of the Gulf Stream encroached upon the 
reef. This specimen, as nearly as I can recall, was about eighteen or twenty 
inches in diameter and is now in the Washington University, St. Louis, Mo. 
Should I live until 1892-93 it is my intention to remove these corals, when I 
shall be glad to give you the exact increase in twenty-five years.” 

Respecting the supply of food for the growing corals of a reef, it is to be 
considered that the amount of life is elsewhere unequalled. Prof. 'T. Fuchs, 
in his paper on the Distribution of Oceanic Life (page 118), speaks of such 
seas within the depth of twenty fathoms as “the gathering ground of an 


> 


extremely rich fauna;” a fauna that embraces “the whole splendor of the 


animal life of the Indian and Pacific Oceans,” and as being of so peculiar 


29 


character that the terms “coral fishes” and “coral mollusks”? would not be 


inappropriate. 


Ill NAMES OF SPECIES IN THE AUTHOR’S REPORT ON 
ZOOPHYTES. 


TuE following catalogue contains the names that are now accepted for the species 
of Actinoid Coral Zoéphytes described in the Author’s Report. The changes have 
chiefly resulted from the subdivision of the old genera. The catalogue has been pre- 


pared for this place by Prof. Verrill, and the explanatory notes have been added by 
him. 


NAMES IN THE AUTHOR’S REPORT. NAMES NOW ACCEPTED, WHEN DIFFERING 


FROM THOSE OF THE REPORT, 


Page 159. Euphyllia pavonina Flabellum pavoninum Lesson. 


<< anthophyllum fe anthophyllum #. & H. 
spheniscus spheniscus #. & IH. 
o rubra = rubrum #. & H. 
4 spinulosa Desmophyllum spinulosum Verridl. 
a glabrescens unchanged. 
Z gracilis Ud 
fs aspera EKusmilia aspera 2. & H. 
oh aperta <c fastigiata H. & H. (?) 
“ costata(p. 720) “« _ costata Verrill. 
ee rugosa unchanged. 
rs turgida cg 
se meandrina Euphyllia fimbriata #. & H. 
sinuosa Pterogyra sinuosa H. & H. 
6 cultrifera “ cultrifera 2. & 71. 
170. Ctenophyllia pectinata (not of Zam.) Pectinia Dane FH. & H. 
quadrata 2 quadrata #. & /I. 
pachyphylla ss pachyphylla #. & H. 
profunda sg profunda 7. & #. 
174. Mussa! fastigiata 
es carduus unchanged. 
. angulosa - 
a corymbosa us 
‘i cactus zl 
z costata 


sinuosa (not of Hillis) Mussa tenuidentata #. & H. 





NAMES OF SPECIES. 421 


NAMES IN THE AUTHOR’S REPORT. 


{74. Mussa cytherea 


“ 


“ce 


“ 


“ 


multilobata 
cerebriformis 
regalis 

crispa (not of Lam.) 


dipsacea 
fragilis 
gyros 
recta 
nobilis 


189. Manicina amarantum 


ce 


“, 


fissa 
areolata * 
meandrites 
hispida 
prerupta 
dilatata 


195. Tridacophyllia lactuca 
196. 


oe 


“ 


pzeonia 
manicina 


198. Caulastrea furcata 


199. 


“ce 


“ 


distorta 
undulata 


205. Orbicella * radiata 


720. 
205. Siderina galaxea 
205. Astrea (Fissicell.) speciosa 


6 


argus 

glaucopis 

patula 

curta 

rotulosa 

coronata 

hyades 

excelsa 

pleiades 

annularis 

stellulata 

stelligera 

crispata 

microplithalma (not of 
Lam.) 

ocellina 

orion 


uva 
ananas 
pandanus 
puteolina 
as pallida 
dipsacea 


NAMES NOW ACCEPTED, WHEN DIFFERING 
FROM THOSE OF THE REPORT. 


unchanged. 


“ec 


« 


“e 


Mussa Indica Verril/, and Mussa ra- 
dians Verrill. 
Isophyllia dipsacea Verrill. 

& fragilis Verrill. 
Colpophyllia gyrosa FE. & H. 
unchanged. 

Tracyphyllia amarantum #, & H. 
Colpophylia. 
Manicina areolata Hhr. 


unchanged. 


Orbicella radiata Dana. 

<c cavernosa Verrill. 

glaucopis Dana. 
Acanthastrea patula 2. & 1 
Plesiastrea curta H. & H. 
Astrea rotulosa Lam. 
Plesiastrea coronata #7. & H. 
Solenastrza hyades Vervrdll. 

Ke excelsa Pourtales 


s pleiades Vervrill. 
Orbicella annularis Dana. J 
€¢ stellulata Dana. 


Plesiastrea stelligera H. & H. 
Ulastrea crispata H#. & H. 


Cyphastrea Danie LH. & J. 

ocellina #. & H. 
Orbicella orion Dana. 
Siderastrea radians Verril. 
Astrea? speciosa Dana. 
Dichocenia uva H. & H. 
Astrea ananas Lam. 

« _ pandanus Dana. 
puteolina Dana. 
pallida Dana. 
Acanthastreea dipsacea Verritll. 


“ 


oe 


29 


NAMES OF SPECIES. 


NAMES IN THE AUTHOR'S REPORT. NAMES NOW ACCEPTED, WHEN DIFFERING 


FROM THOSE OF THE REPORT. 


205. Astrea (Fissicella) porcata (not of 


ce “ee 


“é oe 
6c “6 
“ “ 
“ « 
ce sf 
“e “<< 
aN “ 
“ce « 

“ its 
ii “ 
“ “ 
“ “é 
oe “ce 
“ “é 
“ oe 
“ “ 
ee “<c 
oe “e 
“ it 
“ “ 
oe “ 
“cc “ 
“e “ 
« ti 
« “ 
“ ce 
“ “ 


Esper) Astrea Dane Verrill. 


flexuosa Prionastrza flexuosa Verrill. 
fusco-viridis “ fusco-viridis #. & H. 
virens < virens 1. & H. 
echinata Acanthastrea echinata H. & HA. 
fragilis Astrea fragilis Dana. 

tenella Acanthastrea tenella Verriil. 


magnifica (not 
of Bv.) Prionastrea spectabilis Verrill. 


filicosa Orbicella filicosa Verrill. 
versipora Astrea versipora Lam. 
ff (var.) «  Putnami Verrill. 

denticulata (not 

of Lam.) « cellulosa Verriil. 
pectinata « —_ pectinata Dana. 
deformis Aphrastrea deformis 7. & H. 
var. dedalina Cceloria deedalina Verrill. 
varia Ceeloria spongiosa, var. H. & H. 
rigida Isophyllia rigida Verriit. 


reticularis (not 
of Lam.) Prionastrea Agassizii L. & H. 


petrosa Dichoccenia petrosa Verriil. 
purpurea Leptastrzea purpurea Verrill. 
pulchra ‘Ki pulchra Verrill. 
pentagona Goniastrea pentagona Verriil. 
iavistella « favistella Verrill. 
var., from 

Wakes Il. Astrea Pacifica Verrill. 
eximia Goniastrea eximia 1. & H. 
sinuosa Prionastrea sinuosa Verrill. 
melicerum sf melicerum #. & H. 
parvistella Goniastrea parvistella H. & H. 
favulus Prionastrea favulus Verrill. 
cerium Goniastrea cerium H. & H. 


intersepta (not of 
Lam.) Plesiastrea armata Verrill. 


abdita Prionastreea abdita H. & H. 
tesserifera S tesserifera H. & H. 
robusta se robusta H. & H. 
complanata  (?) oe complanata #7. & H. 
heliopora Orbicella heliopora Vervill. 
“figured Prionastrea valida Verridl. 
Hemprichii Prionastrea Hemprichii #. & H. 
halicora sf halicora H. & H. 
cyclastra Astrea cyclastrea Dana. 
favosa Prionastrea favosa H. & £1. 


254. Meandrina dedalea (not of Hillis) Ceeloria deedalina, var. Ver7idll. 


“cc 


spongiosa ai spongiosa #. & H. 


oe labyrinthica Meandrina labyrinthiformis Verril.. 


NAMES IN THE AUTHOR S REPORT. 


NAMES 


205. Meandrina strigosa 


interrupta 
rustica 
valida 


phrygia (not of Hilzs) 


gracilis 
tenuis 
filograna 
cerebriformis 
truncata 
mammosa 
cylindrus 
caudex 


266 Monticularia microcona 


“e 


“ce 


lobata 
polygonata 


270. Phyllastrea tubifex 
271. Merulina ampliata 


“ 


regalis 
speciosa 
crispa 
folium 
scabricula 
laxa 
rigida 


278. Echinopora undulata 


“ce 


« 


ai 


rosularia 
ringens 
reflexa 
aspera 
horrida 


289. Fungia cyclolites 


“ 


tenuis 

glans 

discus 
agariciformis 
var. tenuifolia 
dentata 


echinata (not of Pal/as) 
var. from Feejees 


repanda 

integra 
confertifolia 
horrida 
actiniformis 
crassitentaculata 
Paumotensis 
dentigera 
scutaria 


OF SPECIES. 4323 


NAMES NOW ACCEPTED, WHEN DIFFERING 
FROM THOSE OF THE REPORT. 


unchanged. 


“e 


Meeandrina rudis Verrill. 
Leptoria gracilis #. & H. 
es tenuis #. & H. 
Meandrina clivosa (young) Verrill 
Diploria cerebriformis H & H. 
B truncata H. & A. 
Meeandrina clivosa Verrili. 
Dendrogyra cylindrus Hhr 
ss caudex Hhr. 
Hydnophora microconos #. & H. 
¢ exesa Hi. & H. 
i polygonata H. & H 
unchanged. 


“ 
[73 
“ 


“e 


Hydnophora Demidoffi Fischer. 
Clavarina scabricula Verriill. 
unchanged. 

Hydnophora rigida #. & H. 
unchanged. 


“e 
“ee 


“e 


Trachypora aspera Verriil. 
Acanthopora horrida Verrilz. 
Cycloseris cyclolites #. & H 


ss tenuis Verrill 
a glans Verrill. 
unchanged. 


Fungia patella H#. dé H. 
Fungia tenuifolia Dana (not HL. & H.) 
unchanged. 
Fungia Dane #. & H. 
« —_lacera Verriil. 
unchanged. 


«““ 


oe 


Lobactis Paumotensis Verrili. 
es Dane Verrill. 
Pleuractis scutaria (Ag. MSS.) Ver- 
rill. 


+ 


4 


NAMES 


NAMES IN THE AUTHOR'S REPORT. 


289. 


307. 


311. 
813. 


319. 
821. 


335. 


Fungia pectinata 
« Ehrenbergii 
« var. gigantea 
“_ asperata 
“ — Ruppellii 
“<"_ erassa 
Herpetolithus limacinus 
Herpetolithus interruptus 
© foliosus 
stellaris 
strictus 
crassus 
Halomitra pileus (not of Linn.) 
Polyphylla talpa 
‘ leptophylla 
sigmoides 
dd pelvis 
5. fungia 
pileiformis 
galeriformis 
Zoépilus echinatus 
Pavonia explanulata 
ad crispa 
oe papyracea 


“cc 


OF SPECIES. 


NAMES NOW ACCEPTED, WHEN DIFFERING 


FROM THOSE OF THE REPORT. 


Ctenactis echinata (Ag. MSS.) Verridl 


“ Ehrenbergii Verrild. 
gigantea Verrill. 
asperata Verriil. 
echinata Verrill. 
crassa Verrill. 
Herpetolitha limax Esch. 


“ 


“ “ce 


unchanged. 
Halomitra clypeus Verrill. 
Cryptabacia talpina 7. & H. 
se leptophylla Z. & A. 
< sigmoides Verrill. 
unchanged. 


Lithactinia pileiformis Z. & H. 


galeriformis #. & A. 


unchanged. 

Podabacia crustacea #. & H. 
Haloseris crispa H. & H. 
Leptoseris papyracea Verriill. 


as elephantotus (not of Pallas) Mycedium elegans &. & H. 


ae cactus unchanged. 

a pretorta x 

6 formosa s 

se venusta se 

os divaricata <s 

“ boletiformis (not of Zam.) Pavonia Dane Verrill. 
< frondifera unchanged. 

= decussata 6 

“ lata 6 


ss crassa 
cs var. loculata 


ae siderea 
Ke latistella 
a clavus 


Agaricia (Undaria) undata 
“ “ 
Lam.) 
speciosa 
levicollis 
planulata 
“ (Mycedia) cucullata 
ss ss purpurea 
fragilis 
gibbosa 
agaricites 


“ec “e 


“ “ 


a “6 


rugosa (not of 


Pavonia loculata Verrill. 
Siderastrea siderea Blainv. 
unchanged. 

Siderastrea clavus Verrill. 
Undaria*® undata Dana. 


‘s monticulosa Verriil. 

« speciosa Dana. 

<s levicollis Dana. 
Asteroseris planulata Vervill. 
Mycedium elephantotus #. & 7 
Agaricia purpurea Les. 
Mycedium fragile Verrild. 
Agaricia gibbosa Dana. 

rs agaricites E. dé H 


ee 
. 


$35. Agaricia (Mycedia) cristata (not of 
345. Psammocora obtusangula 
ee plicata (not of Lam.) 
< fossata 
columna 
sf exesa 
349. Monomyces anthophyllum 
ss eburneus 
870. Cyathina cyathus 
«  pezita 
«s Smithii 
#6 turbinata 
375. Desmophyllum dianthus 
* stellaria 
876. Culicia stellata 
« tenella 
“ truncata 
879. Caryophyllia cespitosa 
< conferta 
i flexuosa 
s arbuscula 
cornigera 
“ anthophyllum 
t solitaria 
sc pocillum 
« dilatata 
385. Dendrophyllia ° ramea 
micrantha 
% nigrescens 
“ aurantiaca 
se coccinea 
“ diaphana 
st rubeola 
oh scabrosa 
391. Oculina hirtella 
ss horrescens 


B29. 


NAMES 


NAMES IN THE AUTHOR'S REPORT. 


oe prolifera 
se axillaris 


varicosa 

s oculata 

“ pallens 

S virginea 

s diffusa 

Anthophyllum musicale 

< fasciculatum 
a astreatum 
f cespitosum 
a hystrix 
“ cuspidatum) 


OF SPECIES. 


425 


NAMES NOW ACCEPTED, WHEN DIFFERING 
FROM THOSE OF THE REPORT. 


Iam.) Agaricia Dane H. & H. 


unchanged. 
Psammocora frondosa Verrili 
unchanged. 


“ 


“se 


Flabellum anthophyllum Z, & H. 
Caryophyllia cyathus Lam. 


“ce 


“ec “e 


< Smithii Stokes. 

Os clavus Scaceni. 
Desmophyllum crista-galli #. & H. (2) 
unchanged. 


Cladocora cespitosa Forbes. 

¢ conferta H. & H. 

‘ stellaria 1. & H. (2) 

ss arbuscula Hdw. 
Dendrophyllia cornigera. 
Lophohelia anthophyllites #. & H. 
Astrangia solitaria Verrill. 
Phyllangia pocillum Verriii. 


unchanged. 


“ce 


“ce 


Dendrophyllia Danze Verrill. 
unchanged. 


Balanophyllia scabrosa Verrtll 
Sclerohelia hirtella H. & H. 
Acrohelia horrescens H. & H. 
Lophohelia prolifera #. & H. 
Cyathohelia axillaris #. & H. 
unchanged. 


unchanged, 

Lophohelia oculata Powrt. 
unchanged. 

Galaxea musicalis Oken. 
fascicularis Oken (in part) 
ss astreata E. & H. 
cespitosa Verrill. 

hystrix Verrtll 

cuspidata Oken. 


426 


NAMES OF SPECIES. i 


NATIES IN THE AUTHOR’S REPORT. 


399. 


404. 
406. 


409. 


414. 


418. 


420. 


423, 


435. 


491. 


Anthophyllum clavus 
Stylina echinulata 
Astroitis calicularis 


ss viridis 

Gemmipora palifera 

ie peltata 

wo patula 

os crater 

i cinerascens 

es frondens 

# brassica 
Astreopora pulvinaria (not of 

Lam.) 

a punctifera 

ee fungiformis 

s stellulata 


Isaura Hemprichii 
“ — Savignii 
«aster 
“speciosa 

Zoantha Ellisii 


re sociata 
Gi Solandri 
ne dubia 


i 


" Bertholetii 
Palythoa denudata 


e auricula 

be nymphea 

< fuliginosa 

s mammillosa 

< ocellata 

¢ glareola 

a flavo-viridis 

yy argus 

‘2 ceesia 
Madrepora 


Names of species unchanged ex- 


cept the following: 

. 238, corymbosa (not of Lam.) 
« 26, plantaginea (not of Lam.) 
« $28, acervata 
« 56, secunda 
« 90, deformis (not of Mich.) 

Manopora 


NAMES NOW ACCEPTED, WHEN DIFFERING 
FROM THOSE OF THE REPORT. * 


Galaxea clavus H. & II. 

unchanged. 

Goniopora viridis H. & H. 

Turbinaria palifera H. & H. 
fs peltata H. & H. 
‘s patula #. & H. 

crater Oken. 

s cinerascens Oken. 

« frondens Verrill. 

sc brassica #7. & H, 


Astreopora profunda Verrill. 

unchanged. 

Turbinaria fungiformis Verrill. 
Us stellulata #. & H. 

unchanged. 


“ 


Zoanthus Ellisii Lam. 

“ sociatus Les. 

‘¢ Solandri Les. 

se dubius Les. 

<< Bertholetii Hhr. 
Mammillifera denudata HAr 


e auricula Les. 
‘ nymphea Les. 
ee fuliginosa Hhr 
unchanged. 
Madrepora. 


Madrepora convexa (young) Dana. 


s appressa (var.) Dana. 

ca plantaginea Lam. 

‘ nobilis (var.) Dana. 

‘s Dane Verrill. 
Montipora. 


Names of species unchanged, except 


the following: 


No. 1, gemmulata 
« 6, crista-galli (not of Hr.) 
« , spumosa (not of Lam.) 


, circumvallata 


Turpmaria gemmulata Verrid. 
Montipora aspera Verrill. 

M. hispida (var.) Dana. 

M. monasteriata H. & H. 


NAMES OF SPECIES. 427 


NAMES IN THE AUTHOR’S REPORT. NAMES NOW ACCEPTED, WHEN DIFFERING 
FROM THOSE OF THE REPORT. 
No. 9, foliosa (not of Paillas) M. Ehrenbergii Verridl. 
“« 21, nudiceps M. crista-galli #. & Z. 
« 25, tuberculosa (not of Lam.) M. Dane F. & H. 
511. Alveopora retepora unchanged. 
“ dedalea (from Red Sea) - 
e ce (specimen fig- 
ured) Alveopora Verrilliana Dana. 
ee spongiosa unchanged. 
f rubra Montipora rubra 7. & H. 
s fenestrata unchanged. - 
*¢ viridis ss , 
515. Sideropora digitata Stylophora digitata #. & H. 
ce elongata ss cs 
6 pistillata ss pistillata Schweigger. 
: subdigitata ée es 
as palmata “ Dane Li. & H. 
a mordax ss mordax Verriil. 
619. Seriatopora subulata anchanged. 
& lineata of 
sf hystrix s 
octoptera oe 
us caliendrum se 
oy var. gracilis Seriatopora gracilis Dana. 
ss valida unchanged. 
523. Pocillopora Pocillipora. 
Names of species unchanged, ex- 
cepting : 
No. 3, brevicornis (var. from Sand- 
wich Is.) Pocillipora cespitosa Dana. 
“6, favosa (var. from Feejees) “s Dane Verrill. 
ge « (var. from Sandwich 
Islands) < aspera Verrill. 
“7, verrucosa (var. from Sand- : 
wich Islands) s avvilis Verril. 
“ 15, plicata (var. from Sandwich 
Islands) e aspera (var. lata) Verrill. 
540. Heliopora coerulea unchanged. : 
§43. Millepora alcicornis o 
ae moniliformis Millepora alcicornis (var.) Linn. 
ns ramosa unchanged. 
pumila % 
iy tortuosa e: 
plicata . 
a complanata A variety of plicata. (?) 
squarrosa unchanged. 
platyphylla <e 
: mordax 4 
va compressa ss 
clavaria My 


flexuosa “ 


428 


NAMES IN THE AUTHOR'S REPORT. 


NAMES OF 


551. Porites furcata 


“ee 


recta 
divaricata 
conferta 
nigrescens 

var. mucronata 
palmata 

levis 

cylindrica 


contigua (not of Hsper) 


astraoides 


SPECIES. 


NAMES NOW ACCEPTED, WHEN DIFFERING 
FROM THOSE OF THE REPORT. 


unchanged, 
Porites furcata (var.) Lam. 
unchanged. 


ce 


ce 


Porites mucronata Daya. 
unchanged, 


«e 


“c 


Synarea Dane Verrill. 
unchanged. 


conglomerata (not of Zam.) Porites lutea @ & H. 


lobata 
fragosa 
limosa 
favosa 
cribripora 
informis 
erosa 
monticulosa 
lichen 
reticulosa 
arenacea 


569. Goniopora pedunculata 


“ce 


“cc 


columna 
Savignyi 


571. Errina aspera 
575 Antipathes 7 spiralis 


oe 


ct% 


anguina 
larix 
eupteridea 
pectinata 


myriophylla 


subpinnata 
reticulata 
flabellum 
ericoides 
mimosella 
pinnatifida 
cupressus 
paniculata 
pennacea 
scoparia 
foeniculum 
corticata 
lacerata 


pyramidata 


Boscii 


alopecuroides 


urborea 


unchanged, 


Synarea informis Verriii. 


6 erosa Verrill. 
6 monticulosa Verriill. 
unchanged. 


“cc 


Porites arenosa H. & H. 
unchanged. 


it9 
“ 
“ 
ce 
&é 
ce 


“e 


Hyalopathes pectinata H. & H. 
unchanged. 


“ 


Hyalopathes corticata 1. & H. 
unchanged. 

Hyalopathes pyramidata 4 & H. 
unchanged. 


“ee 


« 


NAMES OF SPECIES. 429 


NAMES IN THE AUTHOR’S REPORT. NAMES NOW ACCEPTED, WHEN DIFFERING 
FROM THOSE OF THE REPORT. 
575. Antipathes dichotoma unchanged. 
<4 glaberrima Leiopathes glaberrima /. & H. 
fi compressa s compressa FY. « H. 


' The genus, Mussa, as here restricted, includes both Mussa and Symphyllia of 
Milne-Edwards and Haime,—different specimens of the same species sometimes dif- 
fering in the same way, and to the same extent, as do these two so-called genera. The 
only difference given, is dependant upon the mode of growth. 

> It is probable that this, and some of those following it, are only varieties of one 
species. 

3’ The name Orbicella is now restricted to the genus of which 0. annularis and 
O. cavernosa are types. This group is equivalent to Heliastrwa of Edwards and 
Haime, of more recent date. 

* The genus, Astrea, is here restricted to the group of which A. rotulosa is the 
type. This was the original type named by Lamarck, in 1801, when the genus As. 
tr@a was first established. The genus, thus limited, is equivalent to Aawvia of Oken, 
1815. 

5 The genus, Undaria, is equivalent to Pachyseris Edw. and Haime, of later date. 

5 Cenopsammia is recombined with Dendrophyllia, because in certain species 
part of the corallets have the structure of the former genus, and others that of the 
latter, even in the same specimen. ‘The only distinction made is that the former 
genus has a smaller number of lamellz,—a character that is by itself seldom of gen- 
eric value. 

7 The genus, Antipathes, as here adopted, includes Cirrhipathes, Arachnopathes, 
and Rhipidopathes of Edwards and Haime. ‘Those divisions were based only upon 
the modes of growth and branching, which are quite insufficient for establishing gen- 
era among Polyps. 


tN DEX: 


ABACTINAL, 22. ; 
Abrasion and solution in the making of 
lagoon-basins and channels, 293. 
Abrolhos reef, 140, 352. 
Acontia, 54. 
Actinacea, 61. 
Actinal, 22. 
Actinaria, 61. 
Actinia, 20, 22. 
Actinoid Polyps, 21. 
Adamsia palliata, 35. 
Admiralty Islands, 345. 
Africa, reefs of eastern, 350, 351. 
Agaricia agaricites, 99, 113. 
Agassiz, A., on Arachnactis, 28. 
Florida reefs, 204, 285. 
Oahu artesian wells, 287, 417. 
Murray hypothesis, 285. 
effects of currents, 288. 
on solution as a means of making 
lagoon-basins, 295. 
change of level in W. Indies, 304. 
elevated coral rock in Peru, 336. 
‘¢ Seaside Studies,’’ 68, 102. 
‘« Three Cruises of the Blake,’’ 204, 
205. 
Agassiz, L., on Astrangia, 68. 
depth of reef corals, 116. 
coral borers, 121. 
Florida reefs, 204, 205. 
Salt Key Bank, 211. 
Pourtalés Plateau, 211. 
Ahii, 171, 177, 182, 200. 
Aiou, 343. 
Aitutaki, 375. 
Aiva, 264. 


Alcyonacea, 82. 
Alcyonium, derivation of term, 80. 
Alcyonoid Polyps, 21, 80. 
Aldrich, Capt. R. N., on Christmas 
Island, 274, 279. 
on Macclesfield Bank, 271. 
Alexander, W. D., records of artesian 
borings and chart of Oahu received 
from, 411. 
Almirante, 350. 
Alveopora, 75. 
spongiosa, 77. 
Verilliana, 77. 
Anguilla Key, 212. 
Anthea cereus, 37, 57. 
flagellifera, 37. 
Anthelia lineata, 83. 
Antipathacea, 62. 
Antipathes arborea, 63. 
Apaiang, elevation of, 167, 383. 
Apamama, 164, 380. 
Apatite on Mauke, 324. 
Apia, harbor of, 243. 
Aratica, 177, 180, 203, 301. 
Arru Group, 346. 
Artesian borings on Oahu, 287, 411. 
Ascension Island, 392. 
Asia, temperature of ocean along the 
east coast of, 336. 
Asie, 343. 
Astrea ananas, 114. 
gravida, 113. 
pallida, 57, 64. 
Astreeacea, 64. 
distribution of, 109. 
Astrangia Dane, 68. 


432 INDEX. 
Atiu, 194, 341, 372, 398. Bourne, G. C., on Diego Garcia, 279. 
Atlantic Ocean, subsidence in, 403. Bowditch’s Island, 168, 169, 198, 311. 
Atolls, structure of, 162, 174. Branchie in Actiniz, 59. 

origin of, 251, 266. Brazil, corals of, 113. 

origin of lagoons of, 258, 293. reefs, 140, 352. 

proportion of, having entrances to | Brooks’s Island, 360, 378. 

lagoons, 300. Bryozoans, 19, 105. 

completed, 309. Budding in Actiniz, 40. 

submerged, 191, 271. Budding in Coral Polyps, 48. 
Aurora Island, 193. Bunodes gemma, 22. 
Australian reefs, 185, 142, 148, 289, 345,| Byron, of the ‘‘ Blonde,’’ apatite on 

346, 365. Mauke, 324. 

Bauwamas, 214. Caicos group, 215. 

depths near, 173, 216. Calaminianes, 347. 

drift sand rock, 185. Calicle, 42, 44, 48. 
Bailey, J. W., chalk of Oahu, 396. California Gulf, corals of, 112. 
Baker’s Island, 241, 297, 318, 376. Cancrisocia expansa, 24. 
Balbi, on encircling reefs, 266. Cape St. Ann, 351. 
Barrier reefs, origin of, 248, 258, 261. | Carlshoff, 169, 177, 180, 203. 
Beach formations, 184, 221. Caroline Archipelago, 169, 170. 
Beechey, on Henderson Island, 194. elevations in, 380. 

soundings by, 172. Caryophyllia cyathus, 42. 

on Gambier Islands, 266. Smithii, lasso cell, 33. 

on Elizabeth Island, 370. Smithii, animal of, 67. 

on Ducie’s and Osnaburgh Islands, | Carysfort Island, 172. 

371. Cat Island, 215. 

Bermudas, structure of islands, 183, 218. | Caulastrea furcata, 54, 58, 95, Plate IV. 

corals of, 114. Caverns, 194, 224, 392, 397. 

depths near, 173. Celebes, 346. 

drift sand rocks, 185. Ceylon, reefs of, 350. 

former extent of, 408. Cheetetes, 105. 

caverns of, 224. Chagos Bank, 191, 192, 270, 350. 

red earth of, 225. Chalk, origin of, 394. : 

winds of, 225. of Oahu, 399. 
Beveridge reefs, 374. Chamisso, plants of the Marshall Islands, 
Biche-de-Mar, 160. 328. 
Bird Island, 342. Charlotte’s Island, 379. 
Birds of Coral Islands, 329, 330. China, coast of, free from corals, 349. 
Birgi, large crustacea, 198. Christmas Island in Pacific, 178, 375. 
Birnie’s Island, 173, 196, 318, 377. in Indian Ocean, 274, 279. 


Bischof, composition of sea water, 100. | Cladocora arbuscula, 54, 69, 95. 
Bishop, 8. E., on fragments of Coral in | Clarke, H. J., budding in Actiniz, 28. 


Oahu tufas, 417. Clarke, W. B., on Lafu, 344. 
Bland, T., on Bahama and W. India| Classification of Actinoid Polyps, 61. 
mollusk, 407. Clermont Tonnerre, 171, 370. 
Bolabola, 371. Clipperton Rock, 369. 


Bonney, T. G., on Darwin’s theory, 261. | Cnidz, 30. 
Borneo, 347. Cocoanut Grove on Bowditch Island, 311. 


INDEX. 


Cocoanut tree, uses of, 326. 
Coenenchyma, 60. 
Columella, 44, 60. 
Commensalism in polyps, 24, 62. 
Comoro Islands, 350. 
Cook, Captain, on Christmas Island, 
375. 

Cope, E. D., on species of the Anguilla 
caves, 305. 

Cophobelemnon clavatum, 91. 

Corals changed to a phosphate by guano, 
319. 

Corals, rate of growth of, 125, 253, 418. 
temperature limiting, 108, 419. 
influence of impure and fresh wa- 

ters on distribution, 119. 
injured by boring animals, 121. 

Coral, precious, 90. 

Coral heads, 139, 140, 145. 
islands, forms and features, 161. 
submerged, 191, 271. 
poor place for human development, 

332. 
Coral mud and sand of bottom, 142, 
150, 181, 182, 183. 

Coral reefs, rate of growth of, 253. 
benefits from, 159. 
geographical distribution of, 335. 
submarine slopes off reefs, 288. 
forms determined by marine cur- 

rents, 288. 

Coral reef harbors, 160. 
seas, extent of, 336. 

Coral rock, 152, 206, 212, 217, 221, 385. 
solid compact of Metia, 194, 386. 

Coral sands, formation of, 142, 227, 385. 

Coral sand-rocks, 152, 154, 392. 

Corallet, 48, 60. 

Corallide, 90. 

Corallines, 107. 

Corallium from the Sandwich Islands, 91. 

Corallium rubrum, 89. 

Corallum, 42, 48, 60. 
composition of, 98, 105. 
hardness of, 98. 

Corynactics viridis, lasso-cells, 33. 

Coryne, 102. 

Cosmoledo, 350. 

Costz, 160. 

28 


433 


| Couthouy, J. P., on Anthea flagellifera, 
37. 
Crosby, W. O., Cuba elevated reefs, 
Oe 
Ctenactis echinata, 45, 66. 
Cuba, elevated reefs of, 275. 
Currents, as a means of giving atolls 
their features, Guppy, 288, 290. 
A. Agassiz, 288. 
Pacific, 296. 
Cyathophylloids, 21, 78. 


Dati, W. H., Vermetus nigricans on 
Florida shores, 206. 
Dana’s Report on Zodphytes, names of 
species of, 420. 
Danger Island, depths near, 173. 
Darwin, on Coral Reefs, 7, 261. 
depth of reef corals, 115. 
rate of growth of corals, 123. 
origin of coral mud, 231. 
thickness of reefs, 157. 
on soundings, 172. 
on the Maldives, 172, 186, 189, 
268. 
on the Chagos Bank, 192, 270. 
on the Gambier Islands, 157, 265. 
origin of barrier and atoll reefs, 
261. 
geographical distribution of coral 
reefs, 339. 
consolidation of coral sands, 393, 
395. 
objections to theory of, considered, 
Dh 
Dead men’s fingers, 83. 
Dean’s Island, 169, 203, 301, 369. 
Dendrophyllia arborea, 75. 
cornigera, 75. 
nigrescens, 51, 75. 
Depeyster Island, 171, 378. 
Depth of reef-corals, 114, 
Depths near coral reefs, 173, 216, 219, 
288. 
Diego Garcia, 172, 279. 
Diploria cerebriformis, 65, 418. 
Stokesi, 114. 
Disappointment Islands, 172. 
Dissepiments, 60. 


434 


Distribution of corals, 108, 114. 
of coral-reets and islands, 335. 
Dolomite, formation of, 593. 
Dorippe facchino, 24. 
Drift sand-rocks, 154. 
on Oahu, 155. 
Bahamas and Bermudas, 155, 214, 
221. 
of Florida reefs, 185, 206. 
Drift of sands changing with the sea- 
sons at Baker’s Island, 297. 
Drummond’s Island, 167, 314, 315, 379. 
Duchassaing, growth of a Madrepora, 
124. 
Ducie’s Island, 371. 
Duff’s Islands, 344. 
Duke of Clarence’s Islands, 169, 374. 
Duke of York’s Island, 198, 314. 


Eap, 343. 

Easter Island, 339. 

Echinopora reflexa, 43. 

Echthorza, 35. 

Edwards & Haime, Phyllangia Ameri- 
cana, 69. 

Edwardsia callimorpha, 25, 41. 

Egmont Island, 172. 

Elevations in the Pacific, 368. 

Elizabeth Island, an elevated coral isl- 
and, 370, 384. 

Ellice’s Island, 346, 378. 

Ellice group, 301. 

Enderbury’s Island, 182, 186, 196, 577. 

Endotheea, 60. 

Koa, 341. 

Epiactis prolifera, 40. 

Epitheca, 60. 

Eugorgia aurantiaca, 87. 

Eunicella, 87. 

Eupagurus pubescens, 62. 

Evans, Lieut., consolidation of coral 
sands of Ascension Island, 392. 

Exotheca, 60. 

Exuma sound, 216. 


FaKAAFo, 168, 169, 198, 391, 374, 384. 
Fanning Group, 374, 375. 
Favosites, 77, 104. 

relation to Alveopora, 76, 77. 


INDEX. 


Feejees, corals of, 110. 
delta of Rewa, 248. 
reefs of, 262, 341, 363. 
elevations among, 378, 383. 
Whippey harbor, 249. 
Feis, 381. 
Fewkes, origin of lagoon-basin, 303. 
Fission, fissiparity, 56. 
Fissures in reef-rock, 177. 
Fitzroy, Captain, soundings by, 172. 
temperature about the Galapagos, 
336. 
Flabellum spheniscus, 43. 
Flint’s Island, 376. 
Florida Reefs, soundings, etc., 173, 185, 
204, 285. 
rate of growth of corals of, H. T. 
Woodman, 418. 
Florida region, subsidences in, 304. 
former connection with Cuba and 
South America, Heilprin, 306. 
Flustra, 105. 
Foraminifers of reefs, 152. 
Forchammer, magnesia in corals, 99. 
Four Crowns, 199. 
Frigate bird, 331. 
Fringing reefs, 129. 
Fuchs, T., light a cause limiting the 
depth of species, 118. 
rich fauna of coral-reef seas, 419. 
Fungacea, or Fungia tribe, 66. 
distribution of, 109. 
Fungia echinata, 45. 
lacera, 45, 46. 
Dane, 66. 


GALAPAGOS, temperature about, 300. 
Gambier group, 157, 266, 340, 361. 
Gardner’s Island, 377. 
Gemmipora, 75. 
Gente Hermosas, 374. 
Geographical distribution of coral reefs, 
ODD: 
Gilbert Islands, 163, 170, 183, 240, 301, 
$15, 342) 
elevations in, 379, 383. 
toddy of, 327. 
water of, 324. 


Globigerina mud, 143. 


INDEX. 


Goniopora columna, 52, 94, 97. 

Gorgonacea, 89. 

Gorgonia flabellum, 85. 

flexuosa, 85. 
quercifolia, 86. 

Gorgoniz, spicules of, 86. 

Gosse, P. H., species of Peachia, Ed- 
wardsia, etc., 25. 

lasso-cells, 54. 

on Anthea cereus, 37. 

spontaneous fission in Anthea, 57. 

mention of his work, British Sea- 
Anemones, 95. 

Grand Cayman Bank, depths near, 173. 

Growth of corals, rate of, 125, 253, 418. 

Guam, 343. 

elevation of, 381. 
Guano, birds contributing to, 318. 
islands of Pacific, 318. 

Gulf Stream, influence of, in the Juras- 
sic and Cretaceous eras, on the tem- 
perature of the Atlantic Ocean, 400. 

Guppy, H. D., on the Solomon Islands, 
290. 

objections to theory of subsidence, 
288, 290, 291. 
Gypsum on coral islands, 318, 321. 


HagukE, J. D., sands shifted in position 
with the season, 241. 
guano islands of Pacific, 318. 
birds of Pacific coral islands, 530. 
effect of heated air of coral islands 
on winds, 316. 

Hale, H., on Gilbert Islanders, 324. 
on subsidence at Ponape, 367. 

Halocampa chrysanthellum, 25. 

Hapaii Group, 341, 373. 

Haplophyllia paradoxa, 80. 

Harbors and channels, conditions deter- 
mining the formation and condition 
of, 243. 

Hartt, C. F., corals of Brazilian coast, 
115. 

Brazil reefs, 140, 352. 

Hawaiian chain, length of, 401. 
western, coral atolls of, 342, 360, 364. 
northwestern part, soundings in, 173. 
subsidence in, 360. 





435 


Hawaiian Islands, corals of, 111. 
elevations at, 377. 
Hawaii, reefs of, 342. 
Heilprin, A., on the Bermudas, 218, 223, 
225, 304, 307. 
on connection of Florida and South 
America, 306. 
on the views of Mr. Fewkes, 304. 
Heliolites, 94. 
Heliopora, 93. 
Henderson Island, 172, 194. 
Henderville Island, 167. 
Henuake, 167, 182, 199, 329, 369. 
Hero Island, 525, 375. 
Hervey Group, elevations in, 372. 
Hexacoralla, 64. 
Hogoleu, 342. 
Holothuria, dried, 160. 
Honden, see Henuake. 
Hopper Island, 167. 
Horne Island, 542, 378. 
Horsburgh, J. J., on the Maldives, 189. 
Howland’s Island, 319, 376. 
Huahine, shells of, at elevations, 371. 
Hull’s Island, 197, 377. 
Hunt, E. B., rate of growth of corals, 125, 
on Florida Reefs, 204, 207, 288. 
Hunter’s Island, 343. 
Hydra, 101. 
Hydrallmania falcata, 102, 103. 
Hydrocoralline, 70, 103. 
Hydroids, 101, 105. 


INDIAN OCEAN, reefs of, 347. 
subsidence in, 408. 
Ireland Island, Bermudas, 220. 
Isis hippuris, 88. 
Isothermal or isocrymal chart, 108, 335, 
399. 


JAMAICA, elevated reef of, 275. 
Jarvis Island, 168, 316, 319, 375. 
Jones, J. M., on the Bermudas, 218. 





Jukes, Australian reefs, 142, 181. 
Julien, on guano minerals, 323. 


Kao, 341. 
Kauai, 306, 324, 377. 
| Kawehe, 179, 202. 


436 


Keeling’s Island, 172, 261, 350. 


INDEX. 


MAccLESFIELD Bank, 271. 


Kent, W. S., on Veretillum cynomo- | Mackenzie Island, 343, 381. 


rium, 92. 
Key West, 205, 206. 
Kingsmills, see Gilbert Group. 
King’s Island, 200. 
Kophobelemnon, see Cophobelemnon. 
Kotzebue, water of coral islands, 325. 
Kuria, 164, 173, 380. 
Kusaie, 342. 


LaccaDIVvEs, 350. 
Ladrones, 342, 380. 
Lafu, 344, 
Lagoons of atolls, 151, 182, 293, 300, 303. 
Lasso-cells, 30. 
Leconte, Joseph, on Florida Reefs, 204. 
growth of Madrepora, 127. 
Lefroy, on Red Earth of Bermudas, 225. 
Leidy, J., size of a lasso-cell, 33. 
fossil mammals of the West Indies 
and Florida, 306, 308. 
Leptogorgia, 86. 
Leptoria tenuis, Plate IV. 
Level, changes of, in the Pacific, 357, 
368. 
Life and death in concurrent progress, 94. 
Lime in sea-water, 100. 
Limestones, formation of, 385. 
beds of, with living margins, 387. 
thick strata of, 387. 
subsidence essential to the making 
of thick strata of, 387. 
deep sea, from coral reef debris, 
rarely made, 388. 
rate of increase of, 253, 396. 
consolidation of, 591. 
Lisiansky, soundings by, 173. 
on islands northwest of Kauai, 341. 
Lister, J. J., on Christmas Island, 274. 
Lixo coral reef, 140. 
Loculi, 60. 
Logs on coral islands, 196, 316, 317. 
Loochoo, 347. 
Los Guedes, 342. 
Los Matelotas, 342. 
Louisiade Archipelago, 133, 135, 269, 
345. 
Loyalty Group, 344, 381. 


Madagascar reefs, 350. 
Madrepora from a wreck, growth of, 
126, 254. ; 
aspera, 50, 71. 
cervicornis, 99, 1138, 124, 418. 
cribripora, 72, 120. 
formosa, 73. 
palmata, 99, 113. 
prolifera, 115, 124. 

Madreporacea, distribution of, 109. 

Madreporaria, 64. 

Meandrina cerebriformis, see Diploria, 
clivosa, 118, 125, 255, 418. 
labyrinthica, 65, 113, 114, 418. 
sinuosa, 113, 418. 
strigosa, 114. 

Magnesia in corals, 99. 

Magnesian coral limestones, 393. 

Mahlos Mahdoo atoll, 189. 

Maiana, 164, 380. 

Makin, 167, 380. 

Malden’s Island, 291, 375. 

Maldives, 162, 172, 186, 268, 350. 

map of, 187. 

Manicina areolata, 99, 113, 418. 

Mangaia, 274. 
elevation of, 372. 

Manhil, 203. 

Manuing, F. A., analyses of coral sand 

a red earth, 225. 

Manopora, 72. 

Manuai, 373. 

Marakei, 167, 380. 

Margaret, 199. 

Marquesas Key, Florida, 209. 

Islands, 340, 361. 
Marshall Islands, 170, 183, 301, 325, 
328, 380. 

Matea, see Metia. 

Maui, elevation of, 378. 

Mauke, 324, 341, 372. 

McAskill Islands, 342. 

McCandless, artesian borings on Oahu, 

411. 

McKean’s Island, 376. 

Melitza, 89. 

Menchicoff Island, 170, 268. 





INDEX. 


Mendana, 343. 
Merulina, 56, 66, Plate VI. 
Metia, 172, 193, 274, 329, 383, 386. 
magnesian limestone of, 393. 
Metridium marginatum, lasso-cell, 33. 
Millepora alcicornis, 104, 105, 113, 114, 
418. 
Minerals of coral islands, 317. 
Mitiaro, 341. 
Mobius, K., on lasso-cells, 30. 
Molluccas, 346. 
Molokai, elevation of, 377. 
Montipora, 72. 
Moresby, Captain, on the Maldives and 
Chagos Bank, 172, 186, 192. 
Moseley, H. N., on Heliopore, 93. 
on Millipores, 104. 
Mud of channels and lagoons, 142, 150, 
181, 182, 183. 
Muricea, 87. 
Murray, J, on the talus theory and 
Tahiti, 281. 
erosion of emerged lands, 293. 
Mussa, 64. 


Narrsa, 169, 203, 369. 
Namuka, 373. 
Navidad reef, 271. 
Navigator Group, 341, 362, 374. 
Nazareth Bank, 271. 
Necker Island, 342. 
Nelson, on Bermudas, 218. 
on Bahamas, 217. 
Nettling cells, 30. 
New Britain, 345. 
New Caledonia reefs, 135, 148, 344. 
New Guinea, 345. 
New Hebrides, 348, 381. 
New Ireland, 345. 
Newmarket, 377. 
New Zealand Old Hat, 255. 
Nonouti, 164, 169, 380. 
Norfolk Island, 344. 
Nukunono, 374. 
Nullipores, 107, 174. 


Oanu, 306, 324, 411. 
caverns in elevated coral reef of, 398. 
chalk of, 395. 


437 


Oahu, artesian borings on, 287, 411. 
drift sand rock, 155, 417. 
map of part of, 413. 
tufa cones of, 411. 
Oatafu, 198, 374. 
Ocean, depths of, see Depths. 
temperatures of, 335, 399. 
Ocean Island, 362, 378. 
Oceanic currents carry away little detri- 
tus from islands, 143. 
Oceanic subsidence, proofs of, 368, 411. 
Oculina arbuscula, 99. 
diffusa, 114, 125, 256. 
pallens, 114. 
speciosa, 114. 
Valenciennesii, 114. 
varicosa, 69, 114. 
Oculinacea, 66, 109. 
Okatutaia, 341, 372. 
Old Hat, of New Zealand, 235. 
of Anticosti, 236. 
Oolite, 153, 156, 392. 
Oolitic rocks of Florida Keys, 206. 
Orbicella annularis, 113, 125, 418. 
aperta, 113. 
cavernosa, 55, 113, 114, 418. 
Orbicellidee, 67. 
Orbitolites about Australian reefs, 152. 
Organ-pipe Coral, 84. 
Otuhu, 199. 


Paciric, elevations in, 368. 
axis of subsidence in, 363. 
subsidence by broad anticlinals and 
synclinals, 363. 
chain, length of central, 402. 
Palao, see Pelews. 
Pali, 44. 
Palmyra Island, 375. 
Panama, corals of, 111, 339. 
Pandanus, 314, 327. 
Paractis rapiformis, 23. 
Paumotus, 111, 169, 301, 340. 
elevations in, 369, 385. 
botany of, 325. 
Pavonaride, 93. 
Peachia hastata, 25. 
Peacock’s Island, 171, 177, 182, 200. 


| Pearl and Hermes Reef, 361. 


438 


Pelews, 280, 307, 381. 
Pennatulacea, 91. 
Penrhyn’s Island, 375. 
Peritheca, 60, 97. 
Persian Gulf, reefs in, 350. 
Pescadores, 170, 380. 
Phillippine Islands, 348. 
Phoenix Group, depths near, 173, 179. 
on islands of, 195, 376. 
Phyllastraea tubifex, 56, Plate I. 
Phymactis clematis, 22, Plate II. 
florida, 22. 
veratra, 22, Plate II. 
Pitcairn’s Island, 340. 
Plants of Paumotus, list of, 326. 
Plexaura crassa, 114. 
flexuosa, 114. 
homomalla, 114. 
Plexaurella, 87. 
Pliobothrus, 105. 
Plumularia faleata, 102. 
Pocillipora, 70. 
Pocillipora grandis, 71. 
elongata, cell of, 71. 
plicata, cell of, 71. 
Polyps, classification of, 20, 61, 80. 
Ponape, 342 
Porites family, Poritide, 75. 
size of some, 146. 
astreoides, 113, 114, 418. 
clavaria, 113, 114, 418. 
levis, 75, 79. 
mordax, 53, 78. 
solida, 113. 
Port Natal, 351. 
Pourtalés, L. F. de, on Thecocyathus, 43. 
on Haplophyllia, 80. 
rate of growth of corals, 124. 
bottom outside of Florida reefs, 142, 
207, 210. 
depth of reef corals, 116. 
on Florida region, 204. 
Pourtalés Plateau, 211. 
Pouynipete, 342. 
Powell, Lieut., on the Maldives, 186. 
Pterogorgia Americana, 114. 
acerosa, 114. 
Pumice on coral islands, 226, 317. 
Pylstaarts, 341, 374. 


INDEX. 


QUATERNARY changes of level in West 
Indies, 304. 
Quelpaert’s Island, 347. 
Quoy and Gaymard, depth of reef corals, 
115. 
Ascension Island, 351. 


RAIVAVAT, 373. 
Rapa, 340. 
Raraka, 169, 171, 201, 301. 
Rarotonga, 373. 
Red Earth at Bermudas, 225. 
at Tongatabu, 317. 
Red Sea corals, 111, 350. 
Reefs, formation of, 227. 
causes modifying forms of, 242. 
rate of growth of, 253. 
windward side of highest, 240. 
Rein, on Bermudas, 218. 
Renillide, 93. 
Revillagigedo Islands, 339. 
Rice, Wm. N., on the Bermudas, 218, 
221, 224; 295. 
Aimetara, 373. 
Ringgold, Captain, on Penrhyn’s and 
other islands, 376. 
Rivers, effects of, 245. 
Rocks, see Coral. 
Rose Island, 328. 
Rota, elevation of, 381. 
Rotuma, 342, 378. 
Rugosa, 78. 
Rurutu, 340. 
an elevated island, 372. 


SAGARTIA parasitica, 36. 

St. Augustine shell rock, 397. 

Sala-y-Gomez, 340. 

Salomon Islands, see Solomon Islands. 

Salt Key Bank, 210, 211. 

Samoa, see Navigator Group. 

Sandwich Islands, see Hawaiian. 

San Salvador, 216. 

Savage Island, 374. 

| Savali, 862. 

Sawkins, elevated reef of Jamaica, 275. 

Saya-de-Malha, 271. 

Schomburgh, R. H., drift sands of Ane- 
gada, 185. 





INDEX. 


20, 22. 


Sea-anemone, 
Sea-cucumbers, 160. 
Sea, depths of disturbance of, by waves, 
229: 
Sea-ginseng, 160. 
Sea-slugs, 160. 
Sea-water, composition of, 100. 
Semper, Karl, on the Pelews, 289. 
on making of lagoon basins, 294, 298. 
Senses in Actiniz, 39. 
Septa, 43, 60. 
Seriatopora, 70. 
Serle’s Island, 171. 
Serpule in reef-making at Bermudas, 
130,221. 
supposed, of Florida, 206. 
Seychelles, 271. 
Sharples, 8 P., analyses of corals, 99. 
Sherboro Island, 351. 
Shore-platform, origin of, 235. 
Siderastraea radians, 99. 
galaxea, 113. 
radiata, 113. 
Silliman, B., analysis of dolomitic coral 
rock of Metia, 394. 
of coral sand of Straits of Balabac, 
394. 
of chalk of Oahu, 395. 
Society Islands, 340. 
Solomon Islands, 309, 345, 381. 
Somers’ Islands, see Bermudas. 
Sooloo‘Sea, 347. 
Soundings about atolls, 171. 
Spontaneous fission, 56. 
Starbuck's Island, 523, 375. 
Starve Islaad, 325. 
Staver’s Island, 376. 
Stevenson, force of waves, 229. 
Stimpson, Win., observations by, 24, 83, 
84, 92. 
Strombus gigas in West India reefs, 
212217, 
Stutchbury, growth of an Agaricia, 124. 
on Rurutu, 372. 
Stylaster erubescens, 70. 
Stylasteridz, 70, 105, 110. 
Stylophora Danze 70. 
Subsidence theory of coral reefs, 261. 
objections to, 227. 


439 


Subsidence in the Pacific, 357, 401, 411. 
amount of land lost by, 276. 
period and extent of, and accom- 
panying changes over the globe, 
401. 
thickening of reefs by, 387. 
Sunday Island, 341. 
Swain’s Island, 168, 173, 197, 374. 
Swan Island Reef, 175. 
Sydenham Island, 167. 
Sydney Island, 377. 
Synapticule, 60. 


TABUL, 60. 

Tabulate, 60, 77. 

Tafoa, 341, 374. 

Tahiti, north shore of, 149, 246. 
thickness of reef, 158. 
no elevation at, 571. 
Murray’s observations at, 282. 
valleys of, 273. 
arrangements of Wilkes for deter- 

mining rate of growth of reef, 


257, 417. 
same, of Le Clerc and De Benazé, 
ANT. 


Taiara, 167, 200. 
Tampa Bay, 206. 
Tanna, 343. 
‘Tapateuea, 164, 167, 169, 183. 
elevation of, 379. 
Tarawa, 164, 169, 380. 
Tarawan Archipelago, see Gilbert Islands, 
Tari-tari, 167, 169. 
Tealia crassicornis, see Urticina. 
Teku, 199. 
Telesto ramiculosa, 84. 
trichostemma, 83. 
Temperature limiting distribution of 
corals, 108, 335. 
of Atlantic Ocean in past time, 399. 
chart, 335. 
Tetracoralla, 78. 
Thecocyathus cylindraceus, 43. 
Thomson, Sir Wyville, on Bermudas, 
218. 
on red earth, 225. 
Tikopia, 345. 


| Timor, 346. 


44.0 


Timorlaut, 346. 
Tinakoro, 343. 
Tizard Bank, 271. 
Tonga Islands, 341, 373, 384. 
Tongatabu, 341, 373. 
Tortugas, 209. 
Tridacophyllia, 66, Plate 1V. 
Tripang, 160. 
Truk, 342. 
Tubipora fimbriata, 84. 
syringa, 84. 
Tubuai, 340, 373. 
Tubularia, 103. 
Tuomey, M., on Florida reefs, 204, 207. 
Turk’s Islands, 215. 
Tuscarora section in the Pacific, 292. 
Murray on, 292. 
Tutuila, 305, 326. 
Tyerman on Huahine, 371. 
on Rurutu, 372. 


Uatan, 306, 331. 

Umbellularid, 93. 

Upolu, 288, 341, 362. 
thickness of reef, 158, 286. 


harbor of, at Apia, 245; at Falifa, | 


248. 
Urticina crassicornis, lasso-cells, 34, 36. 


VANIKORO Group, 348. 

Vavau, 341, 373, 374. 

Veretillum Stimpsoni, 91, 92. 

cynomorium, phosphorescence of, 
92. 

Vermetus nigricans, Florida, 206. 

Verrill, A. E., on Cancrisocia expansa, 
24; Epiactis prolifera, 40; coral secre- 
tion, 43; classification of actinoid cor- 


als, 61; compartments in Alcyonia all 


ambulacral, 81; Anthelia and Telesto, 
84; spicules of Gorgoniz, 86; species 
of Veretillum and Cophobelemnon, 


91; corals, of Panama, 111; corals of 
La Paz, 112; corals of the West in“| 


INDEX. 


cepted names for species in Dana’s 
Zoophytes, 420. 

Vincennes Island, 179, 202. 

Virgularide, 93. 

Volcanic action limiting the distribu- 
tion of corals, 301, 337, 348. 


Wattis’s IsLanp, 342, 378. 
Washington Island, 173, 199, 374. 
Wateoo, see Atiu. 
Water on coral islands, 324. 
Waterlandt, 205. 
Waves, action of, on coasts, 236, 283. 
force of, Stevenson, 229. 
West Indies, corals of, 112. 
changes of level in, Agassiz, 304; 
Heilprin, 206. 
Weinland, D. F., rate of growth of 
corals, 124. 
Wharton, W. J. L., on Macclesfield 
Bank, 271. 
Whippey Harbor, 250. 
Whipple, J. A., corals from a wreck, 126. 
coral heads of Turk’s Islands, 139. 
Whitsunday Island, 172. 
Williams’s Missionary Enterprises, 341. 
rock and caverns of Atiu, 194, 598. 
on Mangaia, Atiu, and Rurutu, 372. 
Williams, S. W., on biche-de-mar, 161. 
Wilson’s Island, 203. 
| Winds about coral islands, 206, 217, 
223, 316. 
Wolchonsky, 169. 
Woodman, H. T., rate of growth of 
corals, 418, 











XENIA Dana#, 82. 
elongata, 82. 
florida, 82. 


Yap or Eap, 343. 


ZOANTHACEA, 61. 
Zoanthus Americana, 62. 


dies and Brazilian coast, 113; corals | Zodphyte, 48. 


of the Bermudas, 114; Whipple’s ob- | 
servations on corals of a wreck, 126; 


Anticosti shore-platform. 236; 


ac- 


Zoophytes, names now used for species 
in Dana’s report on, Verrill, 420. 
Zoothome, 48, 60. 











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